feat(flutter/player): albumCoverCacheProvider + pass cache through configure

Adds the Riverpod provider (constructed with dioFactory pulling from
the authenticated dioProvider) and extends PlayerActions.playTracks
to forward the cache to audio_handler.configure(). The handler
signature change lands in the next commit.

Intentional intermediate-broken state: audio_handler.configure
doesn't yet accept coverCache. Resolved by the next task in the same
push.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/player/player_provider.dart

@@ -2,13 +2,21 @@ import 'package:audio_service/audio_service.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../auth/auth_provider.dart';
+import '../library/library_providers.dart' show dioProvider;
 import '../models/track.dart';
+import 'album_cover_cache.dart';
 import 'audio_handler.dart';
 
 final audioHandlerProvider = Provider<MinstrelAudioHandler>((ref) {
   throw UnimplementedError('overridden in main()');
 });
 
+final albumCoverCacheProvider = Provider<AlbumCoverCache>((ref) {
+  return AlbumCoverCache(
+    dioFactory: () => ref.read(dioProvider.future),
+  );
+});
+
 final playbackStateProvider = StreamProvider<PlaybackState>(
   (ref) => ref.watch(audioHandlerProvider).playbackState,
 );
@@ -28,7 +36,9 @@ class PlayerActions {
   Future<void> playTracks(List<TrackRef> tracks, {int initialIndex = 0}) async {
     final url   = await _ref.read(serverUrlProvider.future);
     final token = await _ref.read(secureStorageProvider).read(key: 'session_token');
-    final h = _ref.read(audioHandlerProvider)..configure(baseUrl: url ?? '', token: token);
+    final cache = _ref.read(albumCoverCacheProvider);
+    final h = _ref.read(audioHandlerProvider)
+      ..configure(baseUrl: url ?? '', token: token, coverCache: cache);
     await h.setQueueFromTracks(tracks, initialIndex: initialIndex);
     await h.play();
   }