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/features/rooms/presentation/rooms_screen.dart

@@ -0,0 +1,50 @@
+import 'package:flutter/material.dart';
+
+import 'package:household_keeper/l10n/app_localizations.dart';
+
+class RoomsScreen extends StatelessWidget {
+  const RoomsScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final l10n = AppLocalizations.of(context);
+    final theme = Theme.of(context);
+
+    return Center(
+      child: Padding(
+        padding: const EdgeInsets.symmetric(horizontal: 32),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Icon(
+              Icons.door_front_door_rounded,
+              size: 80,
+              color: theme.colorScheme.onSurface.withValues(alpha: 0.4),
+            ),
+            const SizedBox(height: 24),
+            Text(
+              l10n.roomsEmptyTitle,
+              style: theme.textTheme.headlineSmall,
+              textAlign: TextAlign.center,
+            ),
+            const SizedBox(height: 8),
+            Text(
+              l10n.roomsEmptyMessage,
+              style: theme.textTheme.bodyLarge?.copyWith(
+                color: theme.colorScheme.onSurface.withValues(alpha: 0.6),
+              ),
+              textAlign: TextAlign.center,
+            ),
+            const SizedBox(height: 24),
+            FilledButton.tonal(
+              onPressed: () {
+                // Room creation will be implemented in Phase 2
+              },
+              child: Text(l10n.roomsEmptyAction),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}