Add invoice provision-period PDF handling

Normalize invoice provision periods in PDF generation, switch admin labels between single-date and range modes, add end-to-end tests, update package and frontend bundle versions, and note the invoice lifecycle docs update.

Fuchs/Services/InvoiceService.cs

@@ -320,6 +320,22 @@ public class InvoiceService : IInvoiceService
         }
     }
 
+    /// <summary>
+    /// Puts the stored Leistungsdatum / -zeitraum into the PDF admin block: German dates (legacy ISO
+    /// values converted) and the label the editor shows — "Leistungsdatum" for one date,
+    /// "Leistungszeitraum" for a from–to range.
+    /// </summary>
+    internal static void ApplyProvisionPeriod(FuchsPdf.FdsTextBlocks tb, string? provisionPeriod)
+    {
+        tb.ProvisionPeriod = ProvisionPeriodForDisplay(provisionPeriod).Trim();
+        tb.AdminProvLabel  = tb.ProvisionPeriod.Contains(" - ") || tb.ProvisionPeriod.Length == 0
+            ? "Leistungszeitraum" : "Leistungsdatum";
+    }
+
+    /// <summary>Shows ISO dates (yyyy-MM-dd) saved by older editor versions as German dd.MM.yyyy.</summary>
+    internal static string ProvisionPeriodForDisplay(string? period) =>
+        System.Text.RegularExpressions.Regex.Replace(period ?? "", @"\b(\d{4})-(\d{2})-(\d{2})\b", "$3.$2.$1");
+
     public async Task<Document> GenerateInvoicePdfAsync(FdsInvoiceData invoice, bool draft)
     {
         using var act = FuchsTelemetry.StartActivity("invoice.render");
@@ -339,6 +355,7 @@ public class InvoiceService : IInvoiceService
             // e-invoice issue date (ERechnungMapper), see InvoiceDates / ADR 0032.
             AdminDatumValue = InvoiceDates.IssueDate(reg).ToDateTime(TimeOnly.MinValue)
         };
+        ApplyProvisionPeriod(tb, reg?.getString("ProvisionPeriod"));
         var inlineImages = await ResolveInlineImagesAsync(invoice);
         var doc = await _pdf.WriteLetterAsync(tb, draft);
         doc.Info.Title = reg?.getString("InvoiceTitle") ?? "";