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/settings/presentation/settings_screen.dart

@@ -0,0 +1,83 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import 'package:household_keeper/core/theme/theme_provider.dart';
+import 'package:household_keeper/l10n/app_localizations.dart';
+
+class SettingsScreen extends ConsumerWidget {
+  const SettingsScreen({super.key});
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final l10n = AppLocalizations.of(context);
+    final theme = Theme.of(context);
+    final currentThemeMode = ref.watch(themeProvider);
+
+    return ListView(
+      children: [
+        // Section 1: Appearance (Darstellung)
+        Padding(
+          padding: const EdgeInsets.fromLTRB(16, 24, 16, 8),
+          child: Text(
+            l10n.settingsSectionAppearance,
+            style: theme.textTheme.titleMedium?.copyWith(
+              color: theme.colorScheme.primary,
+            ),
+          ),
+        ),
+        ListTile(
+          title: Text(l10n.settingsThemeLabel),
+          subtitle: Padding(
+            padding: const EdgeInsets.only(top: 8),
+            child: SegmentedButton<ThemeMode>(
+              segments: [
+                ButtonSegment<ThemeMode>(
+                  value: ThemeMode.system,
+                  label: Text(l10n.themeSystem),
+                  icon: const Icon(Icons.settings_suggest_outlined),
+                ),
+                ButtonSegment<ThemeMode>(
+                  value: ThemeMode.light,
+                  label: Text(l10n.themeLight),
+                  icon: const Icon(Icons.light_mode_outlined),
+                ),
+                ButtonSegment<ThemeMode>(
+                  value: ThemeMode.dark,
+                  label: Text(l10n.themeDark),
+                  icon: const Icon(Icons.dark_mode_outlined),
+                ),
+              ],
+              selected: {currentThemeMode},
+              onSelectionChanged: (selection) {
+                ref
+                    .read(themeProvider.notifier)
+                    .setThemeMode(selection.first);
+              },
+            ),
+          ),
+        ),
+
+        const Divider(indent: 16, endIndent: 16, height: 32),
+
+        // Section 2: About (Ueber)
+        Padding(
+          padding: const EdgeInsets.fromLTRB(16, 8, 16, 8),
+          child: Text(
+            l10n.settingsSectionAbout,
+            style: theme.textTheme.titleMedium?.copyWith(
+              color: theme.colorScheme.primary,
+            ),
+          ),
+        ),
+        ListTile(
+          title: Text(l10n.aboutAppName),
+          subtitle: Text(l10n.aboutTagline),
+        ),
+        ListTile(
+          title: const Text('Version'),
+          subtitle: Text(l10n.aboutVersion('0.1.0')),
+        ),
+      ],
+    );
+  }
+}