Add audio loading spinner

Summary

• Show a spinning indicator when a track is buffering over a slow connection
• Spinner appears in the track row, tracklist summary, and play/pause button
• Uses the onplaying audio event to clear loading state once playback begins
• Applied to both list view and random view (including crossfade)

Test plan

• Open app on a slow connection (wifi vs ethernet)
• Click a track and verify spinner appears while buffering
• Spinner should replace the stop icon in the track row
• Play/pause button in tracklist header should show spinner
• Collapsed tracklist summary should show spinner next to track name
• Spinner disappears once audio starts playing
• Verify in both list view and random view

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