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/shell/app_shell.dart

@@ -0,0 +1,48 @@
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+
+import 'package:household_keeper/l10n/app_localizations.dart';
+
+class AppShell extends StatelessWidget {
+  const AppShell({
+    required this.navigationShell,
+    super.key,
+  });
+
+  final StatefulNavigationShell navigationShell;
+
+  @override
+  Widget build(BuildContext context) {
+    final l10n = AppLocalizations.of(context);
+
+    return Scaffold(
+      body: navigationShell,
+      bottomNavigationBar: NavigationBar(
+        selectedIndex: navigationShell.currentIndex,
+        onDestinationSelected: (index) {
+          navigationShell.goBranch(
+            index,
+            initialLocation: index == navigationShell.currentIndex,
+          );
+        },
+        destinations: [
+          NavigationDestination(
+            icon: const Icon(Icons.checklist_outlined),
+            selectedIcon: const Icon(Icons.checklist),
+            label: l10n.tabHome,
+          ),
+          NavigationDestination(
+            icon: const Icon(Icons.door_front_door_outlined),
+            selectedIcon: const Icon(Icons.door_front_door),
+            label: l10n.tabRooms,
+          ),
+          NavigationDestination(
+            icon: const Icon(Icons.tune_outlined),
+            selectedIcon: const Icon(Icons.tune),
+            label: l10n.tabSettings,
+          ),
+        ],
+      ),
+    );
+  }
+}