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/main.dart

@@ -1,8 +1,23 @@
+import 'package:audio_service/audio_service.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import 'app.dart';
+import 'player/audio_handler.dart';
+import 'player/player_provider.dart';
 
-void main() {
-  runApp(const ProviderScope(child: MinstrelApp()));
+Future<void> main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  final handler = await AudioService.init(
+    builder: () => MinstrelAudioHandler(),
+    config: const AudioServiceConfig(
+      androidNotificationChannelId: 'com.fabledsword.minstrel.audio',
+      androidNotificationChannelName: 'Minstrel playback',
+      androidNotificationOngoing: true,
+    ),
+  );
+  runApp(ProviderScope(
+    overrides: [audioHandlerProvider.overrideWithValue(handler)],
+    child: const MinstrelApp(),
+  ));
 }