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/pubspec.yaml

@@ -10,22 +10,24 @@ environment:
 dependencies:
   flutter:
     sdk: flutter
-  flutter_riverpod: ^2.5.1
+  flutter_riverpod: ^3.3.1
   dio: ^5.7.0
-  just_audio: ^0.9.40
+  just_audio: ^0.10.5
   audio_service: ^0.18.15
-  flutter_secure_storage: ^9.2.2
-  go_router: ^14.6.1
+  flutter_secure_storage: ^10.1.0
+  go_router: ^17.2.3
   flutter_svg: ^2.0.16
-  google_fonts: ^6.2.1
-  package_info_plus: ^8.0.0
+  google_fonts: ^8.1.0
+  # 10.x conflicts with flutter_secure_storage 10.x on win32. Hold at 8.3.1
+  # until either lib bumps win32 to 6.x.
+  package_info_plus: ^8.3.1
   pub_semver: ^2.1.4
   cupertino_icons: ^1.0.8
 
 dev_dependencies:
   flutter_test:
     sdk: flutter
-  flutter_lints: ^4.0.0
+  flutter_lints: ^6.0.0
   mocktail: ^1.0.4
 
 flutter: