Disable timestamp overlay to fix exit 234 error

Fixed the exit 234 error when generating individual thumbnails by
disabling the timestamp overlay feature which was causing FFmpeg
font-related failures on some systems.

Changes:
- ShowTimestamp: false (was true)
- ShowMetadata: only true for contact sheets (was always true)

The timestamp overlay was causing issues because:
1. DejaVu Sans Mono font might not be available on all systems
2. FFmpeg exits with code 234 when drawtext filter fails
3. Individual thumbnails don't need timestamp overlays anyway

Contact sheets still get metadata headers, which is the main use case
for showing video information on thumbnails.

main.go

@@ -3251,8 +3251,8 @@ func (s *appState) executeThumbJob(ctx context.Context, job *queue.Job, progress
 		ContactSheet:  contactSheet,
 		Columns:       columns,
 		Rows:          rows,
-		ShowTimestamp: true,
-		ShowMetadata:  true,
+		ShowTimestamp: false, // Disabled to avoid font issues
+		ShowMetadata:  contactSheet,
 	}
 
 	result, err := generator.Generate(ctx, config)