feat: add settings page and theme toggle in user dropdown

- Settings page with Appearance, Navigation, and Account sections
- Theme toggle (System/Light/Dark) in profile dropdown
- Theme syncs with API preferences on login
- Default room preference syncs with backend

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/routing/app_router.dart

@@ -4,6 +4,7 @@ import 'package:riverpod_annotation/riverpod_annotation.dart';
 import 'package:tatlock_ui/features/control_room/router.dart';
 import 'package:tatlock_ui/features/front_hall/presentation/pages/front_hall_page.dart';
 import 'package:tatlock_ui/features/security/router.dart';
+import 'package:tatlock_ui/features/settings/presentation/pages/settings_page.dart';
 import 'package:tatlock_ui/shared/layouts/app_scaffold.dart';
 
 part 'app_router.g.dart';
@@ -52,7 +53,7 @@ GoRouter appRouter(Ref ref) {
             path: AppRoutes.settings,
             name: 'settings',
             pageBuilder: (context, state) =>
-                noTransitionPage(context, state, const _PlaceholderPage(title: 'Settings')),
+                noTransitionPage(context, state, const SettingsPage()),
           ),
         ],
       ),