feat(07-02): build SortDropdown widget and integrate into HomeScreen and TaskListScreen

- Create SortDropdown ConsumerWidget using PopupMenuButton<TaskSortOption>
- Displays current sort label with sort icon in AppBar actions
- Check mark shown on active option via Opacity widget
- Add Scaffold with AppBar (title: Übersicht, actions: SortDropdown) to HomeScreen
- Add SortDropdown before edit/delete IconButtons in TaskListScreen AppBar

lib/features/tasks/presentation/task_list_screen.dart

@@ -4,6 +4,7 @@ import 'package:go_router/go_router.dart';
 
 import 'package:household_keeper/core/database/database.dart';
 import 'package:household_keeper/core/providers/database_provider.dart';
+import 'package:household_keeper/features/tasks/presentation/sort_dropdown.dart';
 import 'package:household_keeper/features/tasks/presentation/task_providers.dart';
 import 'package:household_keeper/features/tasks/presentation/task_row.dart';
 import 'package:household_keeper/l10n/app_localizations.dart';
@@ -27,6 +28,7 @@ class TaskListScreen extends ConsumerWidget {
       appBar: AppBar(
         title: _RoomTitle(roomId: roomId),
         actions: [
+          const SortDropdown(),
           IconButton(
             icon: const Icon(Icons.edit),
             onPressed: () => context.go('/rooms/$roomId/edit'),