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/test/player/player_provider_test.dart

@@ -0,0 +1,24 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:minstrel/player/audio_handler.dart';
+import 'package:minstrel/player/player_provider.dart';
+
+void main() {
+  test('audioHandlerProvider must be overridden — bare read throws', () {
+    final container = ProviderContainer();
+    addTearDown(container.dispose);
+    expect(() => container.read(audioHandlerProvider), throwsA(isA<UnimplementedError>()));
+  });
+
+  test('overridden audioHandlerProvider exposes playbackState stream', () {
+    final handler = MinstrelAudioHandler();
+    final container = ProviderContainer(overrides: [
+      audioHandlerProvider.overrideWithValue(handler),
+    ]);
+    addTearDown(container.dispose);
+    expect(container.read(audioHandlerProvider), same(handler));
+    final sub = container.listen(playbackStateProvider, (_, __) {});
+    sub.close();
+  });
+}