feat(07-02): add sort dropdown tests to HomeScreen and fix AppShell test regression

- Add sortPreferenceProvider override to _buildApp test helper
- Add 'HomeScreen sort dropdown' test group: verifies PopupMenuButton<TaskSortOption> and AppBar title
- Fix app_shell_test: expect findsWidgets for 'Übersicht' since AppBar title now also shows it
- 115 tests pass total (113 existing + 2 new)

test/shell/app_shell_test.dart

@@ -63,8 +63,9 @@ void main() {
       // Verify 3 NavigationDestination widgets are rendered
       expect(find.byType(NavigationDestination), findsNWidgets(3));
 
-      // Verify correct German labels from ARB (with umlauts)
-      expect(find.text('\u00dcbersicht'), findsOneWidget);
+      // Verify correct German labels from ARB (with umlauts).
+      // 'Übersicht' appears in both the bottom nav and the HomeScreen AppBar.
+      expect(find.text('\u00dcbersicht'), findsWidgets);
       expect(find.text('R\u00e4ume'), findsOneWidget);
       expect(find.text('Einstellungen'), findsOneWidget);
     });