feat: use German date format (dd.mm.yyyy) with auto-dot insertion

Replace native date picker (mm/dd/yyyy) with a plain text input that
auto-inserts dots as the user types (20122026 → 20.12.2026). A hidden
field transforms the display value back to ISO (yyyy-mm-dd) before
the form is submitted to the backend.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/documents/[id]/edit/+page.svelte

@@ -11,6 +11,38 @@
     let tags = doc.tags ? doc.tags.map((t: any) => t.name) : [];
     let senderId = doc.sender?.id ?? '';
     let selectedReceivers = doc.receivers ?? [];
+
+    function isoToGerman(iso: string): string {
+        if (!iso || !/^\d{4}-\d{2}-\d{2}$/.test(iso)) return '';
+        const [y, m, d] = iso.split('-');
+        return `${d}.${m}.${y}`;
+    }
+
+    function germanToIso(german: string): string {
+        const match = german.match(/^(\d{2})\.(\d{2})\.(\d{4})$/);
+        if (!match) return '';
+        const [, d, m, y] = match;
+        return `${y}-${m}-${d}`;
+    }
+
+    let dateDisplay = isoToGerman(doc.documentDate ?? '');
+    let dateIso = doc.documentDate ?? '';
+
+    function handleDateInput(e: Event) {
+        const input = e.target as HTMLInputElement;
+        const digits = input.value.replace(/\D/g, '').slice(0, 8);
+        let formatted: string;
+        if (digits.length <= 2) {
+            formatted = digits;
+        } else if (digits.length <= 4) {
+            formatted = `${digits.slice(0, 2)}.${digits.slice(2)}`;
+        } else {
+            formatted = `${digits.slice(0, 2)}.${digits.slice(2, 4)}.${digits.slice(4)}`;
+        }
+        input.value = formatted;
+        dateDisplay = formatted;
+        dateIso = germanToIso(formatted);
+    }
 </script>
 
 <div class="max-w-4xl mx-auto py-8 px-4">
@@ -45,11 +77,15 @@
                     <label for="documentDate" class="block text-sm font-medium text-gray-700 mb-1">Datum</label>
                     <input
                         id="documentDate"
-                        type="date"
-                        name="documentDate"
-                        value={doc.documentDate || ''}
+                        type="text"
+                        inputmode="numeric"
+                        value={dateDisplay}
+                        on:input={handleDateInput}
+                        placeholder="TT.MM.JJJJ"
+                        maxlength="10"
                         class="block w-full rounded border-gray-300 shadow-sm p-2 border focus:border-brand-navy focus:ring-brand-navy text-sm"
                     />
+                    <input type="hidden" name="documentDate" value={dateIso} />
                 </div>
 
                 <!-- Ort -->