fix(overlay): drive timing review playhead from mpv playback position

- Preview session observes time-pos, waits for loaded media before seeking, and acks each command
- Forward positions to the modal over media-timing-review:preview-position
- Cursor follows the real position, pauses while buffering, and stays at the clip end when done
- Replace the fixed end-of-clip timer with a 15s stall timeout
- Document preview cursor behavior and ownership in docs

src/renderer/renderer.ts

@@ -625,6 +625,9 @@ function registerModalOpenHandlers(): void {
       mediaTimingReviewModal.handlePreviewEnded(reviewId);
     });
   });
+  window.electronAPI.onMediaTimingReviewPreviewPosition((position) => {
+    mediaTimingReviewModal.handlePreviewPosition(position);
+  });
   window.electronAPI.onOpenPlaylistBrowser(() => {
     runGuardedAsync('playlist-browser:open', async () => {
       await playlistBrowserModal.openPlaylistBrowserModal();