fix(banner): allow motion artwork video to mix with other audio

Enable mixWithOthers on HLS motion header playback so preview audio and
other apps are less likely to be ducked or interrupted.

lib/widgets/motion_header_banner.dart

@@ -56,6 +56,7 @@ class _MotionHeaderBannerState extends State<MotionHeaderBanner>
     final controller = VideoPlayerController.networkUrl(
       Uri.parse(url),
       formatHint: VideoFormat.hls,
+      videoPlayerOptions: VideoPlayerOptions(mixWithOthers: true),
     );
     _controller = controller;
     try {