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/player/audio_handler.dart

@@ -34,8 +34,8 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
       return AudioSource.uri(Uri.parse(url), headers: headers);
     }).toList();
 
-    await _player.setAudioSource(
-      ConcatenatingAudioSource(children: sources),
+    await _player.setAudioSources(
+      sources,
       initialIndex: initialIndex,
     );
   }