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/ios/Runner/Info.plist

@@ -49,6 +49,10 @@
 	</dict>
 	<key>UIApplicationSupportsIndirectInputEvents</key>
 	<true/>
+	<key>UIBackgroundModes</key>
+	<array>
+		<string>audio</string>
+	</array>
 	<key>UILaunchStoryboardName</key>
 	<string>LaunchScreen</string>
 	<key>UIMainStoryboardFile</key>