feat(06-01): build task history sheet, wire into TaskFormScreen, add CalendarTaskRow navigation

- Create task_history_sheet.dart: showTaskHistorySheet() modal bottom sheet
- Sheet uses StreamBuilder on watchCompletionsForTask, shows dates in dd.MM.yyyy + HH:mm format
- Empty state: Icons.history + 'Noch nie erledigt' message
- Count summary shown above list when completions exist
- Add Verlauf ListTile to TaskFormScreen (edit mode only) opening history sheet
- Add onTap to CalendarTaskRow navigating to /rooms/:roomId/tasks/:taskId
- All 106 tests pass, zero analyze issues

lib/features/tasks/presentation/task_form_screen.dart

@@ -10,6 +10,7 @@ import '../../../core/providers/database_provider.dart';
 import '../../../l10n/app_localizations.dart';
 import '../domain/effort_level.dart';
 import '../domain/frequency.dart';
+import 'task_history_sheet.dart';
 import 'task_providers.dart';
 
 /// Full-screen form for task creation and editing.
@@ -186,6 +187,21 @@ class _TaskFormScreenState extends ConsumerState<TaskFormScreen> {
             ),
             const SizedBox(height: 8),
             _buildDueDatePicker(theme),
+
+            // History section (edit mode only)
+            if (widget.isEditing) ...[
+              const SizedBox(height: 24),
+              const Divider(),
+              ListTile(
+                leading: const Icon(Icons.history),
+                title: Text(l10n.taskHistoryTitle),
+                trailing: const Icon(Icons.chevron_right),
+                onTap: () => showTaskHistorySheet(
+                  context: context,
+                  taskId: widget.taskId!,
+                ),
+              ),
+            ],
           ],
         ),
       ),