fix(web): restore known-dimension reservation in ImageEmbed without letterbox fallback

Restores the dimension reservation reverted in dae6f2d, scoped to
the embed.width && embed.height case only. No fallback aspect-ratio
when dims are null — that was the source of the dark letterbox bars
on Tenor/Klipy GIFs and OG-less images that triggered the revert.

The server-side probe in embedResolver.ts:196 already populates dims
for all image-type embeds; this change makes the client honor them.

packages/web/src/components/chat/embeds/ImageEmbed.tsx

@@ -9,17 +9,23 @@ interface ImageEmbedProps {
 export function ImageEmbed({ embed }: ImageEmbedProps) {
   const openImagePreview = useUIStore((s) => s.openImagePreview);
   const imageUrl = embed.image ?? embed.url;
+  const { width, height } = embed;
 
   return (
-    <div className="mt-2 max-w-[400px]">
+    <div className="mt-2 max-w-fit">
+      <div
+        className="relative rounded-lg overflow-hidden"
+        style={width && height ? { aspectRatio: `${width}/${height}`, maxWidth: Math.min(width, 400), maxHeight: 300 } : undefined}
+      >
         <img
           src={imageUrl}
           alt={embed.title ?? ''}
-        className="max-w-full max-h-[300px] rounded-lg cursor-pointer hover:opacity-90 transition-opacity"
+          className="w-full h-full max-w-[400px] max-h-[300px] object-contain rounded-lg cursor-pointer hover:opacity-90 transition-opacity"
           loading="lazy"
           referrerPolicy="no-referrer"
           onClick={() => openImagePreview(imageUrl)}
         />
       </div>
+    </div>
   );
 }