Add player robustness improvements and A/V sync logging

Improvements:
1. Track audio active state with atomic.Bool flag
2. Handle videos without audio track gracefully
   - If audio fails to start, video plays at natural frame rate
   - Clear error messages indicate "video-only playback"
3. Better A/V sync logging for debugging
   - Log when video ahead/behind and actions taken
   - Log good sync status periodically (every ~6 seconds at 30fps)
   - More granular logging for different sync states
4. Proper cleanup when audio stream ends or fails

How it works:
- audioActive flag set to true when audio starts successfully
- Set to false when audio fails to start or ends
- Video checks audioActive before syncing to audio clock
- If no audio: video just paces at natural frame rate (no sync)
- If audio active: full A/V sync with adaptive timing

Expected improvements:
- Video-only files (GIFs, silent videos) play smoothly
- Better debugging info for sync quality
- Graceful degradation when audio missing

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

main.go

@@ -9288,11 +9288,12 @@ func (p *playSession) runVideo(offset float64) {
 				return
 			}
 
-			// Sync video to audio master clock
+			// Sync video to audio master clock (if audio is active)
 			videoFrameTime := offset + (float64(p.frameN) / p.fps)
 			p.videoTime = videoFrameTime
 
-			// Get audio clock time
+			if p.audioActive.Load() {
+				// Audio is active - sync video to audio master clock
 				var audioClockTime float64
 				if audioTimeVal := p.audioTime.Load(); audioTimeVal != nil {
 					audioClockTime = audioTimeVal.(float64)
@@ -9336,6 +9337,10 @@ func (p *playSession) runVideo(offset float64) {
 					}
 					time.Sleep(frameDur)
 				}
+			} else {
+				// No audio - just pace video at its natural frame rate
+				time.Sleep(frameDur)
+			}
 			// Allocate a fresh frame to avoid concurrent texture reuse issues.
 			frame := image.NewRGBA(image.Rect(0, 0, p.targetW, p.targetH))
 			utils.CopyRGBToRGBA(frame.Pix, buf)
@@ -9404,24 +9409,29 @@ func (p *playSession) runAudio(offset float64) {
 		return
 	}
 	if err := cmd.Start(); err != nil {
-		logging.Debug(logging.CatFFMPEG, "audio start failed: %v (%s)", err, strings.TrimSpace(stderr.String()))
+		logging.Debug(logging.CatFFMPEG, "audio start failed (video-only playback): %v (%s)", err, strings.TrimSpace(stderr.String()))
+		p.audioActive.Store(false)
 		return
 	}
 	p.audioCmd = cmd
 	ctx, err := getAudioContext(sampleRate, channels, bytesPerSample)
 	if err != nil {
-		logging.Debug(logging.CatFFMPEG, "audio context error: %v", err)
+		logging.Debug(logging.CatFFMPEG, "audio context error (video-only playback): %v", err)
+		p.audioActive.Store(false)
 		return
 	}
 	player := ctx.NewPlayer()
 	if player == nil {
-		logging.Debug(logging.CatFFMPEG, "audio player creation failed")
+		logging.Debug(logging.CatFFMPEG, "audio player creation failed (video-only playback)")
+		p.audioActive.Store(false)
 		return
 	}
+	p.audioActive.Store(true) // Mark audio as active
 	localPlayer := player
 	go func() {
 		defer cmd.Process.Kill()
 		defer localPlayer.Close()
+		defer p.audioActive.Store(false) // Mark audio as inactive when done
 		// Increased from 4096 (21ms) to 16384 (85ms) for smoother playback
 		// Larger chunks reduce read frequency and improve performance
 		chunk := make([]byte, 16384)