fix: correct frame number expression in trace video

src/api/media_api.rs

@@ -312,9 +312,10 @@ async fn trace_video(
     let mut parts: Vec<String> = Vec::new();
 
     // Global frame number overlay (top-left corner, always visible)
+    let frame_offset = first_frame as i64 - (padding * fps) as i64;
     let global_frame_text = format!(
-        "drawtext=text='Frame\\: %{{n}}+{}':fontsize=16:fontcolor=white:box=1:boxcolor=black@0.6:x=10:y=10",
-        first_frame as i64 - (padding * fps) as i64
+        "drawtext=text='Frame\\: %{{eif:n+{}:d}}':fontsize=16:fontcolor=white:box=1:boxcolor=black@0.6:x=10:y=10",
+        frame_offset
     );
     parts.push(global_frame_text);
     for (i, (frame, x, y, w, h)) in rows.iter().enumerate() {