Fix snippet duration accuracy with stream copy mode

Improves snippet timing accuracy for Default Format mode by:
- Adding -accurate_seek flag for precise keyframe seeking
- Changing from -t (duration) to -to (end time) for better accuracy
- Adding -avoid_negative_ts make_zero to fix timestamp issues with problematic containers like WMV

This should resolve issues where snippets were 1:20 or 0:21 instead of the configured length (e.g., 10s). Stream copy still uses keyframe-level precision but should now be much closer to target duration.

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

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

main.go

@@ -3367,15 +3367,19 @@ func (s *appState) executeSnippetJob(ctx context.Context, job *queue.Job, progre
 	if useSourceFormat {
 		// Source format mode: Use stream copy for clean extraction
 		// Note: This uses keyframe cutting, so duration may not be frame-perfect
+		// Calculate end time for more accurate cutting
+		endTime := center + float64(snippetLength)
 		args = []string{
 			"-y",
 			"-hide_banner",
 			"-loglevel", "error",
+			"-accurate_seek", // Seek accurately to keyframes
 			"-ss", start,
 			"-i", inputPath,
-			"-t", fmt.Sprintf("%d", snippetLength),
-			"-c", "copy", // Stream copy - no re-encoding
-			"-map", "0",  // Include all streams
+			"-to", fmt.Sprintf("%.2f", endTime), // Use -to instead of -t for better accuracy
+			"-c", "copy",                         // Stream copy - no re-encoding
+			"-map", "0",                          // Include all streams
+			"-avoid_negative_ts", "make_zero",    // Fix timestamp issues
 			outputPath,
 		}
 	} else {