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/main.ts

@@ -3009,6 +3009,13 @@ const mediaTimingReviewRuntime = createMediaTimingReviewRuntime({
       }
     }
   },
+  onPreviewPosition: (position) => {
+    for (const window of [overlayManager.getMainWindow(), overlayManager.getModalWindow()]) {
+      if (window && !window.isDestroyed()) {
+        window.webContents.send(IPC_CHANNELS.event.mediaTimingReviewPreviewPosition, position);
+      }
+    }
+  },
   showStatus: (message) =>
     overlayNotificationsRuntime.showConfiguredStatusNotification(message, { variant: 'warning' }),
 });