feat(01-02): create router, navigation shell, screens, and app shell test

- GoRouter with StatefulShellRoute.indexedStack and 3 branches (Home, Rooms, Settings)
- AppShell with NavigationBar using localized labels and thematic icons
- HomeScreen with empty state and cross-navigation to Rooms tab
- RoomsScreen with empty state and placeholder action button
- SettingsScreen with SegmentedButton theme switcher and About section
- Widget test verifying 3 navigation destinations with correct German labels

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

lib/core/router/router.dart

@@ -0,0 +1,42 @@
+import 'package:go_router/go_router.dart';
+
+import 'package:household_keeper/features/home/presentation/home_screen.dart';
+import 'package:household_keeper/features/rooms/presentation/rooms_screen.dart';
+import 'package:household_keeper/features/settings/presentation/settings_screen.dart';
+import 'package:household_keeper/shell/app_shell.dart';
+
+final router = GoRouter(
+  initialLocation: '/',
+  routes: [
+    StatefulShellRoute.indexedStack(
+      builder: (context, state, navigationShell) =>
+          AppShell(navigationShell: navigationShell),
+      branches: [
+        StatefulShellBranch(
+          routes: [
+            GoRoute(
+              path: '/',
+              builder: (context, state) => const HomeScreen(),
+            ),
+          ],
+        ),
+        StatefulShellBranch(
+          routes: [
+            GoRoute(
+              path: '/rooms',
+              builder: (context, state) => const RoomsScreen(),
+            ),
+          ],
+        ),
+        StatefulShellBranch(
+          routes: [
+            GoRoute(
+              path: '/settings',
+              builder: (context, state) => const SettingsScreen(),
+            ),
+          ],
+        ),
+      ],
+    ),
+  ],
+);