Fix player crash: remove premature pollForFile in handleDownloadAndPlay

The unified effect already gates polling on dlStatus===complete, but
handleDownloadAndPlay was still calling pollForFile immediately on click,
before the download even started.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/Watch.jsx

@@ -720,8 +720,7 @@ export default function Watch() {
   const handleDownloadAndPlay = useCallback(() => {
     setPlayRequested(true);
     downloadMut.mutate();
-    pollForFile(`/files/${youtubeVideoId}.mp4`);
-  }, [downloadMut, pollForFile, youtubeVideoId]);
+  }, [downloadMut]);
 
   const saveProgress = useCallback((secs) => {
     if (!video?.id) return;