feat: E-Rechnung (ZUGFeRD) für Nutzer-Rechnungen mit ZIP-Download und E-Mail-Versand, Empfänger-E-Mail und Bankdaten im Profil

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

backend/src/main/resources/messages_en.properties

@@ -1156,3 +1156,19 @@ admincreateinvoices.notification.zugferd.nopositions=No billable positions avail
 admincreateinvoices.notification.zugferd.batcherror=E-invoice {0} could not be created, no invoices were sent: {1}
 admincreateinvoices.notification.zugferd.invalidsend=Validation failed for: {0}. No invoices were sent.
 admincreateinvoices.notification.zip.combined.error=Error creating the combined ZIP file: {0}
+
+# E-invoice (ZUGFeRD) for user invoices to their customers
+invoices.action.zugferd=E-invoice (ZIP)
+invoices.action.sendinvoice=Send by email
+invoices.notification.zugferd.created=E-invoice {0} created
+invoices.notification.zugferd.invalid=E-invoice created, but validation failed. See the validation report in the ZIP file.
+invoices.notification.zugferd.invalidsend=Sending aborted: validation of e-invoice {0} failed.
+invoices.notification.zugferd.error=E-invoice error: {0}
+invoices.notification.email.missing=No email address is stored for the invoice recipient.
+invoices.notification.emailsent=Invoice {0} was sent as an e-invoice to {1}.
+invoices.mail.subject=Your invoice {0}
+invoices.mail.body=Dear Sir or Madam,\n\nPlease find attached your invoice {0} as an e-invoice (ZIP with PDF and invoice data).\n\nKind regards\n{1}
+profile.billing.ustid=VAT ID
+profile.billing.taxnumber=Tax number
+profile.billing.bankname=Bank
+profile.billing.iban=IBAN