fix: remove forced 4/3 aspect ratio and dark background from image containers

Images now size naturally within max constraints instead of being
letterboxed into a 4/3 bg-surface-input container. Fixes dark bars
on GIFs and images whose aspect ratio doesn't match 4/3.

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

@@ -10,16 +10,12 @@ export function ImageEmbed({ embed }: ImageEmbedProps) {
   const openImagePreview = useUIStore((s) => s.openImagePreview);
   const imageUrl = embed.image ?? embed.url;
 
-  const style: React.CSSProperties = embed.width && embed.height
-    ? { aspectRatio: `${embed.width}/${embed.height}`, maxWidth: Math.min(embed.width, 400), maxHeight: 300 }
-    : { aspectRatio: '4/3', maxWidth: 400, maxHeight: 300 };
-
   return (
-    <div className="mt-2 rounded-lg overflow-hidden bg-surface-input" style={style}>
+    <div className="mt-2 max-w-[400px]">
       <img
         src={imageUrl}
         alt={embed.title ?? ''}
-        className="w-full h-full object-contain rounded-lg cursor-pointer hover:opacity-90 transition-opacity"
+        className="max-w-full max-h-[300px] rounded-lg cursor-pointer hover:opacity-90 transition-opacity"
         loading="lazy"
         referrerPolicy="no-referrer"
         onClick={() => openImagePreview(imageUrl)}