feat(flutter): bump deps to latest + Search screen slice

Major version bumps (riverpod 2->3, go_router 14->17, just_audio 0.9->0.10,
flutter_secure_storage 9->10, google_fonts 6->8, flutter_lints 4->6).
package_info_plus held at 8.x due to win32 conflict with secure_storage 10.x.

Riverpod 3 breaks: AsyncValue.valueOrNull -> .value, StateProvider replaced
with Notifier subclass. just_audio 0.10: ConcatenatingAudioSource -> setAudioSources.

Search slice:
- models/page.dart (generic Page<T> envelope)
- models/search_response.dart (3-facet wrapper)
- api/endpoints/search.dart
- search/search_provider.dart (debounced 250ms)
- search/search_screen.dart (TextField + 3 horizontal/vertical sections)
- Wired /search route + search button on home AppBar

flutter_client/lib/shared/routing.dart

@@ -10,6 +10,7 @@ import '../library/artist_detail_screen.dart';
 import '../library/home_screen.dart';
 import '../player/now_playing_screen.dart';
 import '../player/player_bar.dart';
+import '../search/search_screen.dart';
 import 'widgets/version_gate.dart';
 
 /// Exposed as a Provider so its single argument is a real `Ref` (the
@@ -51,6 +52,7 @@ GoRouter buildRouter(Ref ref) {
             builder: (_, s) => AlbumDetailScreen(id: s.pathParameters['id']!),
           ),
           GoRoute(path: '/now-playing', builder: (_, __) => const NowPlayingScreen()),
+          GoRoute(path: '/search', builder: (_, __) => const SearchScreen()),
         ],
       ),
     ],