Improve MPV backend audio track handling

Add proper validation and fallback logic for audio track selection in MPV backend:
- Validate audio track index is within bounds before switching
- Handle streams without separate audio tracks (single asset streams)
- Reset selectedAudioTrackIndex if out of bounds
- Add fallback path for streams without audioTracks array

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Model/Player/Backends/MPVBackend.swift

@@ -326,6 +326,12 @@ final class MPVBackend: PlayerBackend {
                         startPlaying()
                     }
 
+                    // Handle streams with multiple audio tracks
+                    if !stream.audioTracks.isEmpty {
+                        if stream.selectedAudioTrackIndex >= stream.audioTracks.count {
+                            stream.selectedAudioTrackIndex = 0
+                        }
+                        
                         stream.audioAsset = AVURLAsset(url: stream.audioTracks[stream.selectedAudioTrackIndex].url)
                         let fileToLoad = self.model.musicMode ? stream.audioAsset.url : stream.videoAsset.url
                         let audioTrack = self.model.musicMode ? nil : stream.audioAsset.url
@@ -334,6 +340,15 @@ final class MPVBackend: PlayerBackend {
                             self?.isLoadingVideo = true
                             self?.pause()
                         }
+                    } else {
+                        // Fallback for streams without separate audio tracks (e.g., single asset streams)
+                        let fileToLoad = stream.videoAsset.url
+                        
+                        client.loadFile(fileToLoad, bitrate: stream.bitrate, kind: stream.kind, sub: captions?.url, time: time, forceSeekable: stream.kind == .hls) { [weak self] _ in
+                            self?.isLoadingVideo = true
+                            self?.pause()
+                        }
+                    }
                 }
             }
         }
@@ -740,6 +755,12 @@ final class MPVBackend: PlayerBackend {
     func switchAudioTrack(to index: Int) {
         guard let stream, let video else { return }
         
+        // Validate the index is within bounds
+        guard index >= 0 && index < stream.audioTracks.count else {
+            logger.error("Invalid audio track index: \(index), available tracks: \(stream.audioTracks.count)")
+            return
+        }
+
         stream.selectedAudioTrackIndex = index
         model.saveTime { [weak self] in
             self?.playStream(stream, of: video, preservingTime: true, upgrading: false)