refactor: ZIP-Download in AdminCreateInvoicesView auf StreamResource umgestellt

Statt Base64-Data-URL per executeJs wird das ZIP nun wie in InvoicesView
als StreamResource registriert und heruntergeladen — robuster bei großen
Sammel-ZIPs.

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

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

@@ -1,5 +1,6 @@
 package de.assecutor.votianlt.pages.view;
 
+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.dialog.Dialog;
@@ -19,6 +20,8 @@ import com.vaadin.flow.component.textfield.NumberField;
 import com.vaadin.flow.component.textfield.TextField;
 import com.vaadin.flow.router.HasDynamicTitle;
 import com.vaadin.flow.router.Route;
+import com.vaadin.flow.server.StreamRegistration;
+import com.vaadin.flow.server.StreamResource;
 import com.vaadin.flow.theme.lumo.LumoUtility;
 import de.assecutor.votianlt.model.PriceTable;
 import de.assecutor.votianlt.model.User;
@@ -40,6 +43,7 @@ import de.assecutor.votianlt.util.DateTimeFormatUtil;
 import jakarta.annotation.security.RolesAllowed;
 import lombok.extern.slf4j.Slf4j;
 
+import java.io.ByteArrayInputStream;
 import java.math.BigDecimal;
 import java.math.RoundingMode;
 import java.time.LocalDate;
@@ -839,11 +843,12 @@ public class AdminCreateInvoicesView extends Main implements HasDynamicTitle {
 
     /** Offers the given ZIP bytes as browser download. */
     private void triggerZipDownload(byte[] zip, String fileName) {
-        String base64Zip = Base64.getEncoder().encodeToString(zip);
-        getElement().executeJs("const link = document.createElement('a');"
-                + "link.href = 'data:application/zip;base64," + base64Zip + "';" + "link.download = '"
-                + fileName.replace("'", "") + "';"
-                + "document.body.appendChild(link); link.click(); document.body.removeChild(link);");
+        StreamResource resource = new StreamResource(fileName, () -> new ByteArrayInputStream(zip));
+        resource.setContentType("application/zip");
+        resource.setCacheTime(0);
+        StreamRegistration registration = UI.getCurrent().getSession().getResourceRegistry()
+                .registerResource(resource);
+        UI.getCurrent().getPage().open(registration.getResourceUri().toString());
     }
 
     /**