perf(feed): intrinsic dimensions + async decode on video thumbnails

Give the feed card thumbnail width/height (16:9) so the browser reserves the
aspect box before load (CLS), and decoding=async so image decode never blocks
the main thread. loading=lazy was already set.

frontend/src/components/VideoCard.tsx

@@ -153,6 +153,9 @@ function Thumb({
           src={video.thumbnail_url}
           alt=""
           loading="lazy"
+          decoding="async"
+          width={1280}
+          height={720}
           className="w-full h-full object-cover"
         />
       ) : (