feat: E-Rechnungs-Vorschau mit Versand, USt-IdNr.-Validierung und Icon-Buttons

- Rechnungsvorschau (CreateInvoiceView): ZUGFeRD-ZIP wird schon vor der
  Vorschau über das PDF Tool erzeugt und das signierte PDF daraus angezeigt;
  "Speichern" heißt jetzt "Senden" und versendet die Rechnung nach dem
  Speichern als E-Rechnung per E-Mail (Status "Versendet")
- USt-IdNr.-Formatvalidierung (VatIdUtil, EU-Länderformate) in Profil-
  Rechnungsdaten sowie Kunde anlegen/bearbeiten, mit Unit-Tests
- Rechnungsliste: Aktions-Buttons durch Icon-Buttons mit Tooltip ersetzt
- Jobs-Seite: CSV-Export als Icon-Button, Filter greifen automatisch,
  "Filter anwenden"-Button entfernt
- Neue Übersetzungsschlüssel in allen 10 Sprachdateien

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/src/main/java/de/assecutor/votianlt/pages/view/AddCustomerView.java

@@ -63,7 +63,7 @@ public class AddCustomerView extends Main implements HasDynamicTitle {
         ustId.setRequiredIndicatorVisible(true);
         ustId.setPlaceholder("DE123456789");
         ustId.setWidthFull();
-        ustId.addBlurListener(e -> validateField(ustId));
+        ustId.addBlurListener(e -> validateUstId());
 
         // Anrede (Dropdown)
         title = new ComboBox<>(getTranslation("addjob.address.salutation"));
@@ -184,8 +184,10 @@ public class AddCustomerView extends Main implements HasDynamicTitle {
         binder.forField(taxNumber).asRequired(getTranslation("profile.validation.taxnumber.required"))
                 .bind(Customer::getTaxNumber, Customer::setTaxNumber);
 
-        binder.forField(ustId).asRequired(getTranslation("profile.validation.ustid.required")).bind(Customer::getUstId,
-                Customer::setUstId);
+        binder.forField(ustId).asRequired(getTranslation("profile.validation.ustid.required"))
+                .withValidator(de.assecutor.votianlt.util.VatIdUtil::isValid,
+                        getTranslation("profile.validation.ustid.invalid"))
+                .bind(Customer::getUstId, Customer::setUstId);
 
         binder.forField(title).bind(Customer::getTitle, Customer::setTitle);
 
@@ -262,6 +264,24 @@ public class AddCustomerView extends Main implements HasDynamicTitle {
         }
     }
 
+    /**
+     * Prüft die USt-IdNr. auf Pflichtangabe und gültiges Format
+     * (Länderkennzeichen + länderspezifischer Aufbau, z. B. DE123456789).
+     */
+    private void validateUstId() {
+        String value = ustId.getValue();
+        if (value == null || value.trim().isEmpty()) {
+            ustId.setInvalid(true);
+            ustId.setErrorMessage(getTranslation("profile.validation.ustid.required"));
+        } else if (!de.assecutor.votianlt.util.VatIdUtil.isValid(value)) {
+            ustId.setInvalid(true);
+            ustId.setErrorMessage(getTranslation("profile.validation.ustid.invalid"));
+        } else {
+            ustId.setInvalid(false);
+            ustId.setErrorMessage("");
+        }
+    }
+
     private void validateField(TextField field) {
         String value = field.getValue();
         if (value == null || value.trim().isEmpty()) {
@@ -286,7 +306,7 @@ public class AddCustomerView extends Main implements HasDynamicTitle {
 
     private boolean validateAllFields() {
         validateField(taxNumber);
-        validateField(ustId);
+        validateUstId();
         validateField(firstName);
         validateField(lastName);
         validateField(telephone);

backend/src/main/java/de/assecutor/votianlt/pages/view/CreateInvoiceView.java

@@ -33,9 +33,12 @@ import de.assecutor.votianlt.repository.ServiceRepository;
 import de.assecutor.votianlt.repository.UserRepository;
 import de.assecutor.votianlt.security.SecurityService;
 import de.assecutor.votianlt.service.CustomerInvoiceService;
+import de.assecutor.votianlt.service.EmailService;
 import de.assecutor.votianlt.service.InvoiceLifecycleException;
 import de.assecutor.votianlt.service.InvoiceLifecycleService;
 import de.assecutor.votianlt.service.InvoiceTemplateService;
+import de.assecutor.votianlt.service.PdfToolService;
+import de.assecutor.votianlt.service.UserInvoiceZugferdService;
 import jakarta.annotation.security.RolesAllowed;
 import lombok.extern.slf4j.Slf4j;
 import org.bson.types.ObjectId;
@@ -66,6 +69,8 @@ public class CreateInvoiceView extends VerticalLayout implements HasUrlParameter
     private final UserInvoiceDataService userInvoiceDataService;
     private final CustomerService customerService;
     private final InvoiceLifecycleService invoiceLifecycleService;
+    private final UserInvoiceZugferdService userInvoiceZugferdService;
+    private final EmailService emailService;
     private User currentUser;
     private Job currentJob;
     private List<ServiceRow> gridRows = new ArrayList<>();
@@ -120,7 +125,8 @@ public class CreateInvoiceView extends VerticalLayout implements HasUrlParameter
             UserRepository userRepository, CustomerInvoiceService customerInvoiceService,
             InvoiceTemplateService invoiceTemplateService, SecurityService securityService,
             UserInvoiceDataService userInvoiceDataService, CustomerService customerService,
-            InvoiceLifecycleService invoiceLifecycleService) {
+            InvoiceLifecycleService invoiceLifecycleService, UserInvoiceZugferdService userInvoiceZugferdService,
+            EmailService emailService) {
         this.jobRepository = jobRepository;
         this.serviceRepository = serviceRepository;
         this.userRepository = userRepository;
@@ -130,6 +136,8 @@ public class CreateInvoiceView extends VerticalLayout implements HasUrlParameter
         this.userInvoiceDataService = userInvoiceDataService;
         this.customerService = customerService;
         this.invoiceLifecycleService = invoiceLifecycleService;
+        this.userInvoiceZugferdService = userInvoiceZugferdService;
+        this.emailService = emailService;
         setSizeFull();
         setPadding(true);
         setSpacing(true);
@@ -557,7 +565,19 @@ public class CreateInvoiceView extends VerticalLayout implements HasUrlParameter
             byte[] pdfBytes = generateInvoicePdfFromTemplate(templateData, currentUser,
                     getTranslation("createinvoice.preview.number"));
 
-            showPdfPreviewDialog(pdfBytes, templateData, currentUser);
+            // Schon für die Vorschau das ZUGFeRD-ZIP über das PDF Tool erzeugen
+            // und das signierte PDF aus dem ZIP anzeigen — so sieht der Nutzer
+            // exakt das Dokument, das später versendet wird.
+            CustomerInvoice previewInvoice = buildInvoice(getTranslation("createinvoice.preview.number"), pdfBytes,
+                    currentUser);
+            PdfToolService.SignedInvoice signed = userInvoiceZugferdService.sign(previewInvoice);
+            if (!signed.valid()) {
+                Notification.show(getTranslation("invoices.notification.zugferd.invalid"), 7000,
+                        Notification.Position.MIDDLE);
+            }
+            byte[] signedPdf = extractPdfFromZip(signed.zip());
+
+            showPdfPreviewDialog(signedPdf != null ? signedPdf : pdfBytes, templateData, currentUser);
 
         } catch (Exception ex) {
             log.error("Fehler beim Erstellen der Rechnungsvorschau", ex);
@@ -566,36 +586,47 @@ public class CreateInvoiceView extends VerticalLayout implements HasUrlParameter
         }
     }
 
-    private void saveInvoice(String templateData, User user, Dialog previewDialog) {
+    /**
+     * Baut die (noch nicht persistierte) Rechnung mit allen Pflichtangaben nach
+     * §14 UStG auf — auch Grundlage der ZUGFeRD-Metadaten für die E-Rechnung.
+     * Wird sowohl für die Vorschau (Platzhalter-Nummer, transient) als auch für
+     * das endgültige Speichern verwendet.
+     */
+    private CustomerInvoice buildInvoice(String invoiceNumber, byte[] pdfBytes, User user) {
+        BigDecimal netAmount = calculateNetAmount();
+        BigDecimal vatRate = effectiveVatRate();
+        BigDecimal vatAmount = netAmount.multiply(vatRate);
+        BigDecimal totalAmount = netAmount.add(vatAmount);
+
+        CustomerInvoice invoice = new CustomerInvoice();
+        invoice.setInvoiceNumber(invoiceNumber);
+        invoice.setInvoiceDate(java.time.LocalDate.now());
+        invoice.setJobId(currentJob.getId().toHexString());
+        invoice.setUserId(user.getId().toHexString());
+        invoice.setNetAmount(netAmount);
+        invoice.setVatRate(vatRate);
+        invoice.setVatAmount(vatAmount);
+        invoice.setTotalAmount(totalAmount);
+
+        invoice.setDeliveryDate(currentJob.getDeliveryDate() != null ? currentJob.getDeliveryDate()
+                : java.time.LocalDate.now());
+        applySenderData(invoice, user);
+        applyRecipientData(invoice);
+        invoice.setItems(buildInvoiceItems(vatRate));
+
+        invoice.setPdfData(pdfBytes);
+        return invoice;
+    }
+
+    /**
+     * Speichert die Rechnung endgültig und versendet sie anschließend als
+     * signierte ZUGFeRD-E-Rechnung (ZIP) per E-Mail an den Kunden.
+     */
+    private void saveAndSendInvoice(String templateData, User user, Dialog previewDialog) {
         try {
             String invoiceNumber = userInvoiceDataService.generateNextInvoiceNumber(user.getId());
-
-            BigDecimal netAmount = calculateNetAmount();
-            BigDecimal vatRate = effectiveVatRate();
-            BigDecimal vatAmount = netAmount.multiply(vatRate);
-            BigDecimal totalAmount = netAmount.add(vatAmount);
-
             byte[] pdfBytes = generateInvoicePdfFromTemplate(templateData, user, invoiceNumber);
-
-            CustomerInvoice invoice = new CustomerInvoice();
-            invoice.setInvoiceNumber(invoiceNumber);
-            invoice.setInvoiceDate(java.time.LocalDate.now());
-            invoice.setJobId(currentJob.getId().toHexString());
-            invoice.setUserId(user.getId().toHexString());
-            invoice.setNetAmount(netAmount);
-            invoice.setVatRate(vatRate);
-            invoice.setVatAmount(vatAmount);
-            invoice.setTotalAmount(totalAmount);
-
-            // Pflichtangaben nach §14 UStG: Leistungsdatum, Absender, Empfänger und
-            // Positionen — auch Grundlage der ZUGFeRD-Metadaten für die E-Rechnung.
-            invoice.setDeliveryDate(currentJob.getDeliveryDate() != null ? currentJob.getDeliveryDate()
-                    : java.time.LocalDate.now());
-            applySenderData(invoice, user);
-            applyRecipientData(invoice);
-            invoice.setItems(buildInvoiceItems(vatRate));
-
-            invoice.setPdfData(pdfBytes);
+            CustomerInvoice invoice = buildInvoice(invoiceNumber, pdfBytes, user);
 
             // Finalisierung mit Audit-Eintrag und Eindeutigkeitsprüfung der Rechnungsnummer (R-07/R-11/R-36).
             CustomerInvoice savedInvoice = invoiceLifecycleService.createAndIssue(invoice,
@@ -608,6 +639,8 @@ public class CreateInvoiceView extends VerticalLayout implements HasUrlParameter
             Notification.show(getTranslation("createinvoice.notification.saved", invoiceNumber), 4000,
                     Notification.Position.BOTTOM_END);
 
+            sendInvoiceEmail(savedInvoice);
+
         } catch (InvoiceLifecycleException lifecycleEx) {
             log.warn("Lifecycle-Verstoß beim Speichern der Rechnung: {}", lifecycleEx.getMessage());
             Notification.show(lifecycleEx.getMessage(), 5000, Notification.Position.MIDDLE);
@@ -618,6 +651,66 @@ public class CreateInvoiceView extends VerticalLayout implements HasUrlParameter
         }
     }
 
+    /**
+     * Versendet die gespeicherte Rechnung als E-Rechnungs-ZIP per E-Mail.
+     * Besteht die Rechnung die Validierung nicht, wird der Versand abgebrochen —
+     * die Rechnung bleibt gespeichert und kann aus der Rechnungsliste erneut
+     * versendet werden.
+     */
+    private void sendInvoiceEmail(CustomerInvoice savedInvoice) {
+        String recipientEmail = savedInvoice.getRecipientEmail();
+        if (recipientEmail == null || recipientEmail.isBlank()) {
+            Notification.show(getTranslation("invoices.notification.email.missing"), 6000,
+                    Notification.Position.MIDDLE);
+            return;
+        }
+        try {
+            PdfToolService.SignedInvoice signed = userInvoiceZugferdService.sign(savedInvoice);
+            if (!signed.valid()) {
+                log.warn("Versand der Rechnung {} abgebrochen, ZUGFeRD-Validierung fehlgeschlagen",
+                        savedInvoice.getInvoiceNumber());
+                Notification.show(
+                        getTranslation("invoices.notification.zugferd.invalidsend", savedInvoice.getInvoiceNumber()),
+                        7000, Notification.Position.MIDDLE);
+                return;
+            }
+
+            emailService.sendEmailWithAttachment(recipientEmail,
+                    getTranslation("invoices.mail.subject", savedInvoice.getInvoiceNumber()),
+                    getTranslation("invoices.mail.body", savedInvoice.getInvoiceNumber(),
+                            safe(savedInvoice.getSenderName())),
+                    signed.zip(), signed.fileName());
+
+            invoiceLifecycleService.markAsSent(savedInvoice.getId(),
+                    "Als E-Rechnung per E-Mail versendet an " + recipientEmail);
+            Notification.show(
+                    getTranslation("createinvoice.notification.sent", savedInvoice.getInvoiceNumber(), recipientEmail),
+                    5000, Notification.Position.BOTTOM_END);
+        } catch (Exception ex) {
+            log.error("Fehler beim Versand der Rechnung {} an {}: {}", savedInvoice.getInvoiceNumber(), recipientEmail,
+                    ex.getMessage(), ex);
+            Notification.show(getTranslation("invoices.notification.zugferd.error", ex.getMessage()), 7000,
+                    Notification.Position.MIDDLE);
+        }
+    }
+
+    /**
+     * Extrahiert das signierte PDF (PDF/A-3) aus dem vom PDF Tool gelieferten
+     * ZIP; {@code null}, wenn kein PDF-Eintrag enthalten ist.
+     */
+    private byte[] extractPdfFromZip(byte[] zipBytes) throws java.io.IOException {
+        try (java.util.zip.ZipInputStream zip = new java.util.zip.ZipInputStream(
+                new java.io.ByteArrayInputStream(zipBytes))) {
+            java.util.zip.ZipEntry entry;
+            while ((entry = zip.getNextEntry()) != null) {
+                if (!entry.isDirectory() && entry.getName().toLowerCase().endsWith(".pdf")) {
+                    return zip.readAllBytes();
+                }
+            }
+        }
+        return null;
+    }
+
     /**
      * Absenderdaten aus dem Nutzerprofil und den Rechnungs-Stammdaten
      * (USt-IdNr./Steuernummer, Bankverbindung, Zahlungsbedingungen) übernehmen.
@@ -844,21 +937,21 @@ public class CreateInvoiceView extends VerticalLayout implements HasUrlParameter
         Button closeButton = new Button(getTranslation("button.close"), e -> pdfDialog.close());
         closeButton.addThemeVariants(ButtonVariant.LUMO_TERTIARY);
 
-        Button saveButton = new Button(getTranslation("createinvoice.button.save"), e -> {
+        Button sendButton = new Button(getTranslation("createinvoice.button.send"), e -> {
             Dialog confirm = DialogStylingHelper.createConfirmationDialog(
-                    getTranslation("createinvoice.confirm.save.title"),
-                    getTranslation("createinvoice.confirm.save.message"),
+                    getTranslation("createinvoice.confirm.send.title"),
+                    getTranslation("createinvoice.confirm.send.message"),
                     "560px",
                     getTranslation("button.cancel"),
-                    getTranslation("createinvoice.confirm.save.confirm"),
-                    () -> saveInvoice(templateData, user, pdfDialog),
+                    getTranslation("createinvoice.confirm.send.confirm"),
+                    () -> saveAndSendInvoice(templateData, user, pdfDialog),
                     ButtonVariant.LUMO_PRIMARY);
             confirm.open();
         });
-        saveButton.addThemeVariants(ButtonVariant.LUMO_PRIMARY, ButtonVariant.LUMO_SUCCESS);
+        sendButton.addThemeVariants(ButtonVariant.LUMO_PRIMARY, ButtonVariant.LUMO_SUCCESS);
 
         pdfDialog.add(DialogStylingHelper.wrapContent(pdfFrame, true));
-        pdfDialog.getFooter().add(closeButton, saveButton);
+        pdfDialog.getFooter().add(closeButton, sendButton);
         pdfDialog.open();
     }
 

backend/src/main/java/de/assecutor/votianlt/pages/view/EditCustomerView.java

@@ -146,6 +146,8 @@ public class EditCustomerView extends VerticalLayout implements HasUrlParameter<
         binder.forField(taxNumberField).asRequired(getTranslation("profile.validation.taxnumber.required"))
                 .bind(Customer::getTaxNumber, Customer::setTaxNumber);
         binder.forField(ustIdField).asRequired(getTranslation("profile.validation.ustid.required"))
+                .withValidator(de.assecutor.votianlt.util.VatIdUtil::isValid,
+                        getTranslation("profile.validation.ustid.invalid"))
                 .bind(Customer::getUstId, Customer::setUstId);
         binder.forField(firstnameField).bind(Customer::getFirstname, Customer::setFirstname);
         binder.forField(lastNameField).bind(Customer::getLastName, Customer::setLastName);

backend/src/main/java/de/assecutor/votianlt/pages/view/EditProfileView.java

@@ -378,7 +378,7 @@ public class EditProfileView extends HorizontalLayout implements HasDynamicTitle
         ustIdField.setPlaceholder("DE123456789");
         ustIdField.setRequiredIndicatorVisible(true);
         ustIdField.addBlurListener(e -> {
-            validateField(ustIdField, getTranslation("profile.validation.ustid.required"));
+            validateUstIdField();
             saveInvoiceData();
         });
         taxNumberField.setLabel(getTranslation("profile.billing.taxnumber"));
@@ -650,7 +650,7 @@ public class EditProfileView extends HorizontalLayout implements HasDynamicTitle
             // Steuernummer und USt-IdNr. sind Pflicht, sobald die Rechnungslegung
             // aktiviert ist
             if (billingEnabled.getValue()) {
-                validateField(ustIdField, getTranslation("profile.validation.ustid.required"));
+                validateUstIdField();
                 validateField(taxNumberField, getTranslation("profile.validation.taxnumber.required"));
                 isValid = isValid && !ustIdField.isInvalid() && !taxNumberField.isInvalid();
             }
@@ -864,6 +864,24 @@ public class EditProfileView extends HorizontalLayout implements HasDynamicTitle
         return value != null ? value : "";
     }
 
+    /**
+     * Prüft die USt-IdNr. auf Pflichtangabe und gültiges Format
+     * (Länderkennzeichen + länderspezifischer Aufbau, z. B. DE123456789).
+     */
+    private void validateUstIdField() {
+        String value = ustIdField.getValue();
+        if (value == null || value.trim().isEmpty()) {
+            ustIdField.setInvalid(true);
+            ustIdField.setErrorMessage(getTranslation("profile.validation.ustid.required"));
+        } else if (!de.assecutor.votianlt.util.VatIdUtil.isValid(value)) {
+            ustIdField.setInvalid(true);
+            ustIdField.setErrorMessage(getTranslation("profile.validation.ustid.invalid"));
+        } else {
+            ustIdField.setInvalid(false);
+            ustIdField.setErrorMessage("");
+        }
+    }
+
     private void validateField(TextField field, String errorMessage) {
         String value = field.getValue();
         if (value == null || value.trim().isEmpty()) {

backend/src/main/java/de/assecutor/votianlt/pages/view/InvoicesView.java

@@ -4,6 +4,8 @@ import com.vaadin.flow.component.Component;
 import com.vaadin.flow.component.UI;
 import com.vaadin.flow.component.button.Button;
 import com.vaadin.flow.component.button.ButtonVariant;
+import com.vaadin.flow.component.icon.Icon;
+import com.vaadin.flow.component.icon.VaadinIcon;
 import com.vaadin.flow.component.dialog.Dialog;
 import com.vaadin.flow.component.grid.Grid;
 import com.vaadin.flow.component.html.Anchor;
@@ -186,13 +188,15 @@ public class InvoicesView extends VerticalLayout implements HasDynamicTitle {
         actions.setSpacing(true);
         actions.setPadding(false);
 
-        Button viewBtn = new Button(getTranslation("invoices.action.view"), e -> downloadInvoicePdf(invoice));
-        viewBtn.addThemeVariants(ButtonVariant.LUMO_TERTIARY, ButtonVariant.LUMO_SMALL);
+        Button viewBtn = actionIconButton(VaadinIcon.EYE, "invoices.action.view",
+                e -> downloadInvoicePdf(invoice));
+        viewBtn.addThemeVariants(ButtonVariant.LUMO_TERTIARY);
         viewBtn.setEnabled(invoice.getPdfData() != null && invoice.getPdfData().length > 0);
         actions.add(viewBtn);
 
-        Button historyBtn = new Button(getTranslation("invoices.action.history"), e -> openHistoryDialog(invoice));
-        historyBtn.addThemeVariants(ButtonVariant.LUMO_TERTIARY, ButtonVariant.LUMO_SMALL);
+        Button historyBtn = actionIconButton(VaadinIcon.TIME_BACKWARD, "invoices.action.history",
+                e -> openHistoryDialog(invoice));
+        historyBtn.addThemeVariants(ButtonVariant.LUMO_TERTIARY);
         actions.add(historyBtn);
 
         InvoiceStatus status = invoice.getStatus() != null ? invoice.getStatus() : InvoiceStatus.ISSUED;
@@ -205,40 +209,38 @@ public class InvoicesView extends VerticalLayout implements HasDynamicTitle {
 
         if (type == InvoiceType.INVOICE && status == InvoiceStatus.ISSUED
                 && invoicePermissionService.canMarkAsSent(currentUser)) {
-            Button sentBtn = new Button(getTranslation("invoices.action.marksent"),
+            Button sentBtn = actionIconButton(VaadinIcon.CHECK, "invoices.action.marksent",
                     e -> markAsSent(invoice));
-            sentBtn.addThemeVariants(ButtonVariant.LUMO_SMALL);
             actions.add(sentBtn);
         }
 
         // E-Rechnung (ZUGFeRD) über das externe PDF Tool — analog zu den
         // System-Rechnungen der Admin-Seite
         if (type == InvoiceType.INVOICE && invoice.getPdfData() != null && invoice.getPdfData().length > 0) {
-            Button zugferdBtn = new Button(getTranslation("invoices.action.zugferd"),
+            Button zugferdBtn = actionIconButton(VaadinIcon.FILE_ZIP, "invoices.action.zugferd",
                     e -> downloadZugferdZip(invoice));
-            zugferdBtn.addThemeVariants(ButtonVariant.LUMO_TERTIARY, ButtonVariant.LUMO_SMALL);
+            zugferdBtn.addThemeVariants(ButtonVariant.LUMO_TERTIARY);
             actions.add(zugferdBtn);
 
             if (isLiveInvoice && invoicePermissionService.canMarkAsSent(currentUser)) {
-                Button sendBtn = new Button(getTranslation("invoices.action.sendinvoice"),
+                Button sendBtn = actionIconButton(VaadinIcon.PAPERPLANE, "invoices.action.sendinvoice",
                         e -> sendAsEInvoice(invoice));
-                sendBtn.addThemeVariants(ButtonVariant.LUMO_SMALL, ButtonVariant.LUMO_PRIMARY);
+                sendBtn.addThemeVariants(ButtonVariant.LUMO_PRIMARY);
                 actions.add(sendBtn);
             }
         }
 
         if (isLiveInvoice) {
             if (invoicePermissionService.canCorrect(currentUser)) {
-                Button correctBtn = new Button(getTranslation("invoices.action.correct"),
+                Button correctBtn = actionIconButton(VaadinIcon.EDIT, "invoices.action.correct",
                         e -> openCorrectionDialog(invoice));
-                correctBtn.addThemeVariants(ButtonVariant.LUMO_SMALL);
                 actions.add(correctBtn);
             }
 
             if (invoicePermissionService.canCancel(currentUser)) {
-                Button cancelBtn = new Button(getTranslation("invoices.action.cancel"),
+                Button cancelBtn = actionIconButton(VaadinIcon.BAN, "invoices.action.cancel",
                         e -> openCancellationDialog(invoice));
-                cancelBtn.addThemeVariants(ButtonVariant.LUMO_SMALL, ButtonVariant.LUMO_ERROR);
+                cancelBtn.addThemeVariants(ButtonVariant.LUMO_ERROR);
                 actions.add(cancelBtn);
             }
         }
@@ -246,20 +248,33 @@ public class InvoicesView extends VerticalLayout implements HasDynamicTitle {
         // Zahlung erfassen: nur für reguläre Rechnungen (R-25)
         if (type == InvoiceType.INVOICE && status != InvoiceStatus.DRAFT
                 && invoicePermissionService.canRecordPayment(currentUser)) {
-            Button payBtn = new Button(getTranslation("invoices.action.payment"),
+            Button payBtn = actionIconButton(VaadinIcon.EURO, "invoices.action.payment",
                     e -> openPaymentDialog(invoice));
-            payBtn.addThemeVariants(ButtonVariant.LUMO_SMALL);
             actions.add(payBtn);
         }
 
         // Belegpaket exportieren (R-33/R-34)
-        Button exportBtn = new Button(getTranslation("invoices.action.export"), e -> exportPackage(invoice));
-        exportBtn.addThemeVariants(ButtonVariant.LUMO_TERTIARY, ButtonVariant.LUMO_SMALL);
+        Button exportBtn = actionIconButton(VaadinIcon.DOWNLOAD, "invoices.action.export",
+                e -> exportPackage(invoice));
+        exportBtn.addThemeVariants(ButtonVariant.LUMO_TERTIARY);
         actions.add(exportBtn);
 
         return actions;
     }
 
+    /**
+     * Icon-Button für die Aktionsspalte: die bisherige Button-Beschriftung
+     * bleibt als Tooltip (Bubblehelp) und Aria-Label erhalten.
+     */
+    private Button actionIconButton(VaadinIcon icon, String labelKey,
+            com.vaadin.flow.component.ComponentEventListener<com.vaadin.flow.component.ClickEvent<Button>> listener) {
+        Button button = new Button(new Icon(icon), listener);
+        button.addThemeVariants(ButtonVariant.LUMO_SMALL, ButtonVariant.LUMO_ICON);
+        button.setTooltipText(getTranslation(labelKey));
+        button.setAriaLabel(getTranslation(labelKey));
+        return button;
+    }
+
     private void openPaymentDialog(CustomerInvoice invoice) {
         Dialog dialog = DialogStylingHelper.createStyledDialog(
                 getTranslation("invoices.payment.title", invoice.getInvoiceNumber()), "480px");

backend/src/main/java/de/assecutor/votianlt/pages/view/ShowJobsView.java

@@ -70,8 +70,10 @@ public class ShowJobsView extends VerticalLayout implements HasDynamicTitle {
         endDate.setLabel(getTranslation("jobs.filter.enddate"));
         searchField.setLabel(getTranslation("jobs.filter.search"));
         statusFilter.setLabel(getTranslation("jobs.filter.status"));
-        Button exportButton = new Button(getTranslation("jobs.button.csvexport"));
+        Button exportButton = new Button(new Icon(VaadinIcon.DOWNLOAD));
         exportButton.addThemeVariants(ButtonVariant.LUMO_TERTIARY);
+        exportButton.setTooltipText(getTranslation("jobs.button.csvexport"));
+        exportButton.setAriaLabel(getTranslation("jobs.button.csvexport"));
         ViewToolbar toolbar = new ViewToolbar(getTranslation("nav.jobs"), exportButton);
         add(toolbar);
 
@@ -85,9 +87,7 @@ public class ShowJobsView extends VerticalLayout implements HasDynamicTitle {
         searchField.setClearButtonVisible(true);
 
         // Filterleiste mit Export-Button am rechten Rand
-        Button applyFilter = new Button(getTranslation("jobs.filter.apply"));
-        applyFilter.addThemeVariants(ButtonVariant.LUMO_PRIMARY);
-        HorizontalLayout leftFilters = new HorizontalLayout(startDate, endDate, searchField, statusFilter, applyFilter);
+        HorizontalLayout leftFilters = new HorizontalLayout(startDate, endDate, searchField, statusFilter);
         leftFilters.setAlignItems(Alignment.END);
         leftFilters.getStyle().set("flex-wrap", "wrap");
 
@@ -105,7 +105,6 @@ public class ShowJobsView extends VerticalLayout implements HasDynamicTitle {
         java.time.LocalDate today = java.time.LocalDate.now();
         startDate.setValue(today.minusDays(30));
         endDate.setValue(today);
-        applyFilter.addClickListener(e -> loadData());
         exportButton.addClickListener(e -> exportToCsv());
 
         // Add real-time filtering

backend/src/main/java/de/assecutor/votianlt/util/VatIdUtil.java

@@ -0,0 +1,81 @@
+package de.assecutor.votianlt.util;
+
+import java.util.Map;
+import java.util.regex.Pattern;
+
+/**
+ * Formatvalidierung für Umsatzsteuer-Identifikationsnummern (USt-IdNr.).
+ *
+ * Prüft den Aufbau nach Länderkennzeichen gemäß den offiziellen Formaten der
+ * EU-Mitgliedstaaten (z. B. DE + 9 Ziffern). Für Länderkennzeichen ohne
+ * hinterlegtes Muster wird ein generisches Format (2 Buchstaben + 2–12
+ * alphanumerische Zeichen) akzeptiert. Es findet keine Online-Prüfung (VIES)
+ * statt — validiert wird ausschließlich das Format.
+ */
+public final class VatIdUtil {
+
+    /** Offizielle USt-IdNr.-Formate der EU-Mitgliedstaaten (ohne Länderpräfix). */
+    private static final Map<String, Pattern> COUNTRY_PATTERNS = Map.ofEntries(
+            Map.entry("AT", Pattern.compile("U\\d{8}")),
+            Map.entry("BE", Pattern.compile("[01]\\d{9}")),
+            Map.entry("BG", Pattern.compile("\\d{9,10}")),
+            Map.entry("CY", Pattern.compile("\\d{8}[A-Z]")),
+            Map.entry("CZ", Pattern.compile("\\d{8,10}")),
+            Map.entry("DE", Pattern.compile("\\d{9}")),
+            Map.entry("DK", Pattern.compile("\\d{8}")),
+            Map.entry("EE", Pattern.compile("\\d{9}")),
+            Map.entry("EL", Pattern.compile("\\d{9}")),
+            Map.entry("ES", Pattern.compile("[A-Z0-9]\\d{7}[A-Z0-9]")),
+            Map.entry("FI", Pattern.compile("\\d{8}")),
+            Map.entry("FR", Pattern.compile("[A-Z0-9]{2}\\d{9}")),
+            Map.entry("HR", Pattern.compile("\\d{11}")),
+            Map.entry("HU", Pattern.compile("\\d{8}")),
+            Map.entry("IE", Pattern.compile("\\d{7}[A-Z]{1,2}|\\d[A-Z0-9+*]\\d{5}[A-Z]")),
+            Map.entry("IT", Pattern.compile("\\d{11}")),
+            Map.entry("LT", Pattern.compile("(\\d{9}|\\d{12})")),
+            Map.entry("LU", Pattern.compile("\\d{8}")),
+            Map.entry("LV", Pattern.compile("\\d{11}")),
+            Map.entry("MT", Pattern.compile("\\d{8}")),
+            Map.entry("NL", Pattern.compile("\\d{9}B\\d{2}")),
+            Map.entry("PL", Pattern.compile("\\d{10}")),
+            Map.entry("PT", Pattern.compile("\\d{9}")),
+            Map.entry("RO", Pattern.compile("\\d{2,10}")),
+            Map.entry("SE", Pattern.compile("\\d{12}")),
+            Map.entry("SI", Pattern.compile("\\d{8}")),
+            Map.entry("SK", Pattern.compile("\\d{10}")));
+
+    /** Rückfallmuster für Länderkennzeichen ohne hinterlegtes Format. */
+    private static final Pattern GENERIC_PATTERN = Pattern.compile("[A-Z]{2}[A-Z0-9]{2,12}");
+
+    private VatIdUtil() {
+    }
+
+    /**
+     * Prüft, ob der Wert eine formal gültige USt-IdNr. ist. Leerzeichen,
+     * Punkte und Bindestriche werden vor der Prüfung entfernt,
+     * Kleinbuchstaben akzeptiert. {@code null} oder leer gilt als ungültig.
+     */
+    public static boolean isValid(String vatId) {
+        String normalized = normalize(vatId);
+        if (normalized == null || normalized.length() < 4) {
+            return false;
+        }
+        String countryCode = normalized.substring(0, 2);
+        if (!countryCode.chars().allMatch(Character::isLetter)) {
+            return false;
+        }
+        Pattern pattern = COUNTRY_PATTERNS.get(countryCode);
+        if (pattern != null) {
+            return pattern.matcher(normalized.substring(2)).matches();
+        }
+        return GENERIC_PATTERN.matcher(normalized).matches();
+    }
+
+    /** Entfernt Trennzeichen und wandelt in Großbuchstaben um. */
+    public static String normalize(String vatId) {
+        if (vatId == null) {
+            return null;
+        }
+        return vatId.replaceAll("[\\s.\\-]", "").toUpperCase(java.util.Locale.ROOT);
+    }
+}

backend/src/main/resources/messages_de.properties

@@ -86,6 +86,7 @@ profile.validation.zip.required=Postleitzahl ist erforderlich
 profile.validation.city.required=Stadt ist erforderlich
 profile.validation.taxnumber.required=Steuernummer ist erforderlich
 profile.validation.ustid.required=USt-IdNr. ist erforderlich
+profile.validation.ustid.invalid=Ungültiges Format der USt-IdNr. (z. B. DE123456789)
 profile.validation.firstname.required=Vorname ist erforderlich
 profile.validation.lastname.required=Nachname ist erforderlich
 profile.validation.phone.required=Telefonnummer ist erforderlich
@@ -699,6 +700,11 @@ createinvoice.button.save=Speichern
 createinvoice.confirm.save.title=Rechnung speichern
 createinvoice.confirm.save.message=Diese Rechnung wird dauerhaft gespeichert und kann anschlie\u00dfend nicht mehr ver\u00e4ndert werden. Fortfahren?
 createinvoice.confirm.save.confirm=Ja, speichern
+createinvoice.button.send=Senden
+createinvoice.confirm.send.title=Rechnung senden
+createinvoice.confirm.send.message=Diese Rechnung wird dauerhaft gespeichert und als E-Rechnung (ZIP) per E-Mail an den Kunden versendet. Fortfahren?
+createinvoice.confirm.send.confirm=Ja, senden
+createinvoice.notification.sent=Rechnung {0} wurde als E-Rechnung an {1} versendet
 
 # Invoices
 invoices.title=Rechnungen

backend/src/main/resources/messages_ee.properties

@@ -75,6 +75,7 @@ profile.validation.zip.required=Postiindeks on kohustuslik
 profile.validation.city.required=Linn on kohustuslik
 profile.validation.taxnumber.required=Maksunumber on kohustuslik
 profile.validation.ustid.required=KMKR nr on kohustuslik
+profile.validation.ustid.invalid=KMKR numbri vorming on vale (nt DE123456789)
 profile.validation.firstname.required=Eesnimi on kohustuslik
 profile.validation.lastname.required=Perekonnanimi on kohustuslik
 profile.validation.phone.required=Telefoninumber on kohustuslik
@@ -639,6 +640,11 @@ createinvoice.button.save=Salvesta
 createinvoice.confirm.save.title=Arve salvestamine
 createinvoice.confirm.save.message=See arve salvestatakse p\u00fcsivalt ja seda ei saa hiljem muuta. Kas j\u00e4tkata?
 createinvoice.confirm.save.confirm=Jah, salvesta
+createinvoice.button.send=Saada
+createinvoice.confirm.send.title=Saada arve
+createinvoice.confirm.send.message=Arve salvestatakse jäädavalt ja saadetakse kliendile e-posti teel e-arvena (ZIP). Kas jätkata?
+createinvoice.confirm.send.confirm=Jah, saada
+createinvoice.notification.sent=Arve {0} saadeti e-arvena aadressile {1}
 invoices.title=Arved
 invoices.column.number=Number
 invoices.column.customer=Klient

backend/src/main/resources/messages_en.properties

@@ -86,6 +86,7 @@ profile.validation.zip.required=Postal code is required
 profile.validation.city.required=City is required
 profile.validation.taxnumber.required=Tax number is required
 profile.validation.ustid.required=VAT ID is required
+profile.validation.ustid.invalid=Invalid VAT ID format (e.g. DE123456789)
 profile.validation.firstname.required=First name is required
 profile.validation.lastname.required=Last name is required
 profile.validation.phone.required=Phone number is required
@@ -699,6 +700,11 @@ createinvoice.button.save=Save
 createinvoice.confirm.save.title=Save Invoice
 createinvoice.confirm.save.message=This invoice will be permanently saved and cannot be modified afterwards. Continue?
 createinvoice.confirm.save.confirm=Yes, save
+createinvoice.button.send=Send
+createinvoice.confirm.send.title=Send Invoice
+createinvoice.confirm.send.message=This invoice will be permanently saved and sent to the customer by email as an e-invoice (ZIP). Continue?
+createinvoice.confirm.send.confirm=Yes, send
+createinvoice.notification.sent=Invoice {0} has been sent as an e-invoice to {1}
 
 # Invoices
 invoices.title=Invoices

backend/src/main/resources/messages_es.properties

@@ -85,6 +85,7 @@ profile.validation.zip.required=C\u00f3digo postal es obligatorio
 profile.validation.city.required=Ciudad es obligatorio
 profile.validation.taxnumber.required=Número fiscal es obligatorio
 profile.validation.ustid.required=NIF-IVA es obligatorio
+profile.validation.ustid.invalid=Formato de NIF-IVA no válido (p. ej. DE123456789)
 profile.validation.firstname.required=Nombre es obligatorio
 profile.validation.lastname.required=Apellido es obligatorio
 profile.validation.phone.required=N\u00famero de tel\u00e9fono es obligatorio
@@ -696,6 +697,11 @@ createinvoice.button.save=Guardar
 createinvoice.confirm.save.title=Guardar factura
 createinvoice.confirm.save.message=Esta factura se guardar\u00e1 de forma permanente y no podr\u00e1 ser modificada posteriormente. \u00bfDesea continuar?
 createinvoice.confirm.save.confirm=S\u00ed, guardar
+createinvoice.button.send=Enviar
+createinvoice.confirm.send.title=Enviar factura
+createinvoice.confirm.send.message=La factura se guardará de forma permanente y se enviará al cliente por correo electrónico como factura electrónica (ZIP). ¿Continuar?
+createinvoice.confirm.send.confirm=Sí, enviar
+createinvoice.notification.sent=La factura {0} se ha enviado como factura electrónica a {1}
 
 # Invoices
 invoices.title=Facturas

backend/src/main/resources/messages_fr.properties

@@ -85,6 +85,7 @@ profile.validation.zip.required=Le code postal est obligatoire
 profile.validation.city.required=La ville est obligatoire
 profile.validation.taxnumber.required=Le numéro fiscal est obligatoire
 profile.validation.ustid.required=Le n° TVA intracommunautaire est obligatoire
+profile.validation.ustid.invalid=Format de numéro de TVA invalide (p. ex. DE123456789)
 profile.validation.firstname.required=Le pr\u00e9nom est obligatoire
 profile.validation.lastname.required=Le nom est obligatoire
 profile.validation.phone.required=Le num\u00e9ro de t\u00e9l\u00e9phone est obligatoire
@@ -696,6 +697,11 @@ createinvoice.button.save=Enregistrer
 createinvoice.confirm.save.title=Enregistrer la facture
 createinvoice.confirm.save.message=Cette facture sera enregistr\u00e9e de mani\u00e8re permanente et ne pourra plus \u00eatre modifi\u00e9e par la suite. Continuer ?
 createinvoice.confirm.save.confirm=Oui, enregistrer
+createinvoice.button.send=Envoyer
+createinvoice.confirm.send.title=Envoyer la facture
+createinvoice.confirm.send.message=La facture sera enregistrée définitivement et envoyée au client par e-mail en tant que facture électronique (ZIP). Continuer ?
+createinvoice.confirm.send.confirm=Oui, envoyer
+createinvoice.notification.sent=La facture {0} a été envoyée en tant que facture électronique à {1}
 
 # Invoices
 invoices.title=Factures

backend/src/main/resources/messages_lt.properties

@@ -85,6 +85,7 @@ profile.validation.zip.required=Pašto kodas yra privalomas
 profile.validation.city.required=Miestas yra privalomas
 profile.validation.taxnumber.required=Mokesčių mokėtojo numeris yra privalomas
 profile.validation.ustid.required=PVM mokėtojo kodas yra privalomas
+profile.validation.ustid.invalid=Neteisingas PVM mokėtojo kodo formatas (pvz., DE123456789)
 profile.validation.firstname.required=Vardas yra privalomas
 profile.validation.lastname.required=Pavardė yra privaloma
 profile.validation.phone.required=Telefono numeris yra privalomas
@@ -698,6 +699,11 @@ createinvoice.button.save=Išsaugoti
 createinvoice.confirm.save.title=Išsaugoti sąskaitą
 createinvoice.confirm.save.message=Ši sąskaita bus išsaugota visam laikui ir vėliau jos nebebus galima keisti. Tęsti?
 createinvoice.confirm.save.confirm=Taip, išsaugoti
+createinvoice.button.send=Siųsti
+createinvoice.confirm.send.title=Siųsti sąskaitą
+createinvoice.confirm.send.message=Sąskaita bus visam laikui išsaugota ir išsiųsta klientui el. paštu kaip e. sąskaita (ZIP). Tęsti?
+createinvoice.confirm.send.confirm=Taip, siųsti
+createinvoice.notification.sent=Sąskaita {0} išsiųsta kaip e. sąskaita adresu {1}
 
 # Invoices
 invoices.title=Sąskaitos faktūros

backend/src/main/resources/messages_lv.properties

@@ -85,6 +85,7 @@ profile.validation.zip.required=Pasta indekss ir obligāts
 profile.validation.city.required=Pilsēta ir obligāta
 profile.validation.taxnumber.required=Nodokļu maksātāja numurs ir obligāts
 profile.validation.ustid.required=PVN numurs ir obligāts
+profile.validation.ustid.invalid=Nederīgs PVN numura formāts (piem., DE123456789)
 profile.validation.firstname.required=Vārds ir obligāts
 profile.validation.lastname.required=Uzvārds ir obligāts
 profile.validation.phone.required=Tālruņa numurs ir obligāts
@@ -696,6 +697,11 @@ createinvoice.button.save=Saglabāt
 createinvoice.confirm.save.title=Saglabāt rēķinu
 createinvoice.confirm.save.message=Šis rēķins tiks neatgriezeniski saglabāts un pēc tam to vairs nevarēs mainīt. Turpināt?
 createinvoice.confirm.save.confirm=Jā, saglabāt
+createinvoice.button.send=Sūtīt
+createinvoice.confirm.send.title=Sūtīt rēķinu
+createinvoice.confirm.send.message=Rēķins tiks neatgriezeniski saglabāts un nosūtīts klientam pa e-pastu kā e-rēķins (ZIP). Vai turpināt?
+createinvoice.confirm.send.confirm=Jā, sūtīt
+createinvoice.notification.sent=Rēķins {0} nosūtīts kā e-rēķins uz {1}
 
 # Invoices
 invoices.title=Rēķini

backend/src/main/resources/messages_pl.properties

@@ -85,6 +85,7 @@ profile.validation.zip.required=Kod pocztowy jest wymagany
 profile.validation.city.required=Miasto jest wymagane
 profile.validation.taxnumber.required=Numer podatkowy jest wymagany
 profile.validation.ustid.required=NIP UE jest wymagany
+profile.validation.ustid.invalid=Nieprawidłowy format numeru VAT UE (np. DE123456789)
 profile.validation.firstname.required=Imi\u0119 jest wymagane
 profile.validation.lastname.required=Nazwisko jest wymagane
 profile.validation.phone.required=Numer telefonu jest wymagany
@@ -696,6 +697,11 @@ createinvoice.button.save=Zapisz
 createinvoice.confirm.save.title=Zapisz faktur\u0119
 createinvoice.confirm.save.message=Ta faktura zostanie trwale zapisana i nie b\u0119dzie mog\u0142a by\u0107 p\u00f3\u017aniej zmieniona. Kontynuowa\u0107?
 createinvoice.confirm.save.confirm=Tak, zapisz
+createinvoice.button.send=Wyślij
+createinvoice.confirm.send.title=Wyślij fakturę
+createinvoice.confirm.send.message=Faktura zostanie trwale zapisana i wysłana do klienta e-mailem jako e-faktura (ZIP). Kontynuować?
+createinvoice.confirm.send.confirm=Tak, wyślij
+createinvoice.notification.sent=Faktura {0} została wysłana jako e-faktura na adres {1}
 
 # Invoices
 invoices.title=Faktury

backend/src/main/resources/messages_ru.properties

@@ -85,6 +85,7 @@ profile.validation.zip.required=Почтовый индекс обязателе
 profile.validation.city.required=Город обязателен
 profile.validation.taxnumber.required=Налоговый номер обязателен
 profile.validation.ustid.required=Идентификатор плательщика НДС обязателен
+profile.validation.ustid.invalid=Неверный формат идентификационного номера НДС (напр. DE123456789)
 profile.validation.firstname.required=Имя обязательно
 profile.validation.lastname.required=Фамилия обязательна
 profile.validation.phone.required=Номер телефона обязателен
@@ -696,6 +697,11 @@ createinvoice.button.save=Сохранить
 createinvoice.confirm.save.title=Сохранить счёт
 createinvoice.confirm.save.message=Этот счёт будет сохранён навсегда и не может быть изменён впоследствии. Продолжить?
 createinvoice.confirm.save.confirm=Да, сохранить
+createinvoice.button.send=Отправить
+createinvoice.confirm.send.title=Отправить счёт
+createinvoice.confirm.send.message=Счёт будет окончательно сохранён и отправлен клиенту по электронной почте как электронный счёт (ZIP). Продолжить?
+createinvoice.confirm.send.confirm=Да, отправить
+createinvoice.notification.sent=Счёт {0} отправлен как электронный счёт на {1}
 
 # Invoices
 invoices.title=Счета

backend/src/main/resources/messages_tr.properties

@@ -85,6 +85,7 @@ profile.validation.zip.required=Posta kodu gereklidir
 profile.validation.city.required=\u015eehir gereklidir
 profile.validation.taxnumber.required=Vergi numarası gereklidir
 profile.validation.ustid.required=KDV kimlik no gereklidir
+profile.validation.ustid.invalid=Geçersiz KDV kimlik numarası biçimi (örn. DE123456789)
 profile.validation.firstname.required=Ad gereklidir
 profile.validation.lastname.required=Soyad gereklidir
 profile.validation.phone.required=Telefon numaras\u0131 gereklidir
@@ -696,6 +697,11 @@ createinvoice.button.save=Kaydet
 createinvoice.confirm.save.title=Faturay\u0131 Kaydet
 createinvoice.confirm.save.message=Bu fatura kal\u0131c\u0131 olarak kaydedilecek ve sonras\u0131nda de\u011fi\u015ftirilemeyecektir. Devam edilsin mi?
 createinvoice.confirm.save.confirm=Evet, kaydet
+createinvoice.button.send=Gönder
+createinvoice.confirm.send.title=Faturayı gönder
+createinvoice.confirm.send.message=Fatura kalıcı olarak kaydedilecek ve müşteriye e-posta ile e-fatura (ZIP) olarak gönderilecektir. Devam edilsin mi?
+createinvoice.confirm.send.confirm=Evet, gönder
+createinvoice.notification.sent=Fatura {0}, e-fatura olarak {1} adresine gönderildi
 
 # Invoices
 invoices.title=Faturalar

backend/src/test/java/de/assecutor/votianlt/util/VatIdUtilTest.java

@@ -0,0 +1,45 @@
+package de.assecutor.votianlt.util;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+class VatIdUtilTest {
+
+    @ParameterizedTest
+    @ValueSource(strings = { "DE123456789", "ATU12345678", "NL123456789B01", "FRXX123456789", "PL1234567890",
+            "IT12345678901", "BE0123456789", "SE123456789012", "IE1234567AB" })
+    void validVatIdsAreAccepted(String vatId) {
+        assertTrue(VatIdUtil.isValid(vatId), vatId + " sollte gültig sein");
+    }
+
+    @Test
+    void separatorsAndLowercaseAreNormalized() {
+        assertTrue(VatIdUtil.isValid("de 123.456-789"));
+        assertEquals("DE123456789", VatIdUtil.normalize("de 123.456-789"));
+    }
+
+    @ParameterizedTest
+    @ValueSource(strings = { "DE12345678", "DE1234567890", "DE12345678A", "ATU1234567", "NL123456789A01",
+            "123456789", "D123456789", "XX", "DE" })
+    void invalidVatIdsAreRejected(String vatId) {
+        assertFalse(VatIdUtil.isValid(vatId), vatId + " sollte ungültig sein");
+    }
+
+    @Test
+    void nullAndBlankAreInvalid() {
+        assertFalse(VatIdUtil.isValid(null));
+        assertFalse(VatIdUtil.isValid(""));
+        assertFalse(VatIdUtil.isValid("   "));
+    }
+
+    @Test
+    void unknownCountryFallsBackToGenericPattern() {
+        assertTrue(VatIdUtil.isValid("XI123456789"));
+        assertFalse(VatIdUtil.isValid("XI1"));
+    }
+}