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/models/page.dart

@@ -0,0 +1,42 @@
+// Mirrors the server's Page<T> envelope used by paged endpoints
+// (/api/search, /api/library/artists, /api/library/albums,
+// /api/library/history, /api/me/likes/*).
+//
+// Wire shape from internal/api/types.go Page[T]:
+//   { items: T[], total: int, limit: int, offset: int }
+//
+// Dart generics can't auto-infer T from JSON, so callers pass an
+// itemFromJson function alongside the raw map. Parse logic stays in
+// each entity's own fromJson; this class only handles the envelope.
+class Page<T> {
+  const Page({
+    required this.items,
+    required this.total,
+    required this.limit,
+    required this.offset,
+  });
+
+  final List<T> items;
+  final int total;
+  final int limit;
+  final int offset;
+
+  factory Page.fromJson(
+    Map<String, dynamic> j,
+    T Function(Map<String, dynamic>) itemFromJson,
+  ) {
+    final raw = (j['items'] as List?) ?? const [];
+    return Page<T>(
+      items: raw
+          .map((e) => itemFromJson((e as Map).cast<String, dynamic>()))
+          .toList(growable: false),
+      total: (j['total'] as num?)?.toInt() ?? 0,
+      limit: (j['limit'] as num?)?.toInt() ?? 0,
+      offset: (j['offset'] as num?)?.toInt() ?? 0,
+    );
+  }
+
+  /// Convenience for endpoints that always return the first page or
+  /// when the caller just wants the items.
+  bool get hasMore => offset + items.length < total;
+}

flutter_client/lib/models/search_response.dart

@@ -0,0 +1,38 @@
+import 'album.dart';
+import 'artist.dart';
+import 'page.dart';
+import 'track.dart';
+
+/// Mirrors internal/api/search.go SearchResponse — three pages keyed by
+/// facet, each independently paged. The mobile UI usually only walks
+/// the first page of each facet (limit 20-50); infinite scroll within a
+/// facet is a future enhancement, not v1.
+class SearchResponse {
+  const SearchResponse({
+    required this.artists,
+    required this.albums,
+    required this.tracks,
+  });
+
+  final Page<ArtistRef> artists;
+  final Page<AlbumRef> albums;
+  final Page<TrackRef> tracks;
+
+  factory SearchResponse.fromJson(Map<String, dynamic> j) => SearchResponse(
+        artists: Page.fromJson(
+          (j['artists'] as Map?)?.cast<String, dynamic>() ?? const {},
+          ArtistRef.fromJson,
+        ),
+        albums: Page.fromJson(
+          (j['albums'] as Map?)?.cast<String, dynamic>() ?? const {},
+          AlbumRef.fromJson,
+        ),
+        tracks: Page.fromJson(
+          (j['tracks'] as Map?)?.cast<String, dynamic>() ?? const {},
+          TrackRef.fromJson,
+        ),
+      );
+
+  bool get isEmpty =>
+      artists.items.isEmpty && albums.items.isEmpty && tracks.items.isEmpty;
+}