feat(flutter/player): MinstrelAudioHandler + just_audio + audio_service

audio_service runs the handler in a background isolate; UI watches
playbackState/mediaItem/queue streams through Riverpod. AndroidManifest
gets the foreground-service media-playback permission; Info.plist gets
UIBackgroundModes=audio. Bearer token attached to stream URLs via
just_audio's per-source headers.

flutter_client/lib/player/player_provider.dart

@@ -0,0 +1,37 @@
+import 'package:audio_service/audio_service.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../auth/auth_provider.dart';
+import '../models/track.dart';
+import 'audio_handler.dart';
+
+final audioHandlerProvider = Provider<MinstrelAudioHandler>((ref) {
+  throw UnimplementedError('overridden in main()');
+});
+
+final playbackStateProvider = StreamProvider<PlaybackState>(
+  (ref) => ref.watch(audioHandlerProvider).playbackState,
+);
+
+final mediaItemProvider = StreamProvider<MediaItem?>(
+  (ref) => ref.watch(audioHandlerProvider).mediaItem,
+);
+
+final queueProvider = StreamProvider<List<MediaItem>>(
+  (ref) => ref.watch(audioHandlerProvider).queue,
+);
+
+class PlayerActions {
+  PlayerActions(this._ref);
+  final Ref _ref;
+
+  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);
+    await h.setQueueFromTracks(tracks, initialIndex: initialIndex);
+    await h.play();
+  }
+}
+
+final playerActionsProvider = Provider<PlayerActions>((ref) => PlayerActions(ref));