This commit addresses crashes caused by race conditions when accessing audio track arrays:
- MPVBackend.swift: Use safe index clamping to prevent array out of bounds crashes when selecting audio tracks
- PlayerModel.swift: Add selectedAudioTrack computed property for thread-safe audio track access
- ControlsOverlay.swift: Use safe accessor with "Original" fallback label
- PlaybackSettings.swift: Use safe accessor with "Original" fallback label
This fix resolves approximately 37% of crashes (23 out of 62 crash logs) that were caused by index out of range errors in MPVBackend.playStream at line 345.
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