fix annotation offset seeking in tracking details (#24936)

The VOD manifest used to find the keyframe-snapped clip start was built from the live annotation offset while the playing clip used the offset from when the event opened. Each offset step fetched a manifest for a different clip, so the seek mapping drifted: the video jumped forward about one GOP every second of offset and flickered while each new manifest loaded. The manifest now uses the same stable source offset as the playlist

web/src/components/overlay/detail/TrackingDetails.tsx

@@ -150,17 +150,18 @@ export function TrackingDetails({
 
   // Fetch the VOD manifest JSON to get the actual clipFrom after keyframe
   // snapping. The backend may snap clipFrom backwards to a keyframe, making
-  // the video start earlier than the requested time.
+  // the video start earlier than the requested time. Uses sourceOffsetRef so
+  // the manifest describes the same clip as videoSource.
   const vodManifestUrl = useMemo(() => {
     if (!event.camera) return null;
-    const startTime =
-      event.start_time + annotationOffset / 1000 - REVIEW_PADDING;
+    const sourceOffset = sourceOffsetRef.current;
+    const startTime = event.start_time + sourceOffset / 1000 - REVIEW_PADDING;
     const endTime =
       (event.end_time ?? Date.now() / 1000) +
-      annotationOffset / 1000 +
+      sourceOffset / 1000 +
       REVIEW_PADDING;
     return `vod/clip/${event.camera}/start/${startTime}/end/${endTime}`;
-  }, [event, annotationOffset]);
+  }, [event]);
 
   const { data: vodManifest } = useSWR<VodManifest>(vodManifestUrl, null, {
     revalidateOnFocus: false,