Add avoid_negative_ts make_zero for progressive MP4 downloads

- Ensures timestamps start at 0 for better player compatibility
- Fixes timestamp/seeking issues in mpv and other strict players

MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs

@@ -7495,7 +7495,8 @@ namespace MediaBrowser.Controller.MediaEncoding
                 {
                     // Progressive download - use faststart for proper seeking and duration
                     // Use frag_keyframe for better seeking compatibility with mpv
-                    format = " -f mp4 -movflags frag_keyframe+faststart+default_base_moof";
+                    // avoid_negative_ts make_zero ensures timestamps start at 0 for better player compatibility
+                    format = " -f mp4 -movflags frag_keyframe+faststart+default_base_moof -avoid_negative_ts make_zero";
                 }
             }