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/home/presentation/calendar_task_row.dart

@@ -36,6 +36,9 @@ class CalendarTaskRow extends StatelessWidget {
     final task = taskWithRoom.task;
 
     return ListTile(
+      onTap: () => context.go(
+        '/rooms/${taskWithRoom.roomId}/tasks/${taskWithRoom.task.id}',
+      ),
       leading: Checkbox(
         value: false,
         onChanged: (_) => onCompleted(),