fix: resolve 4 media embed bugs from testing

1. Video aspect ratio: remove container border/overflow-hidden, use
   preload="metadata" so browser knows dimensions before play
2. YouTube Error 153: remove sandbox attr (too restrictive), add full
   allow permissions (encrypted-media, accelerometer, gyroscope, etc.)
3. Google Images not displaying: detect image Content-Type from HTTP
   response in metadataFetcher, override classifier to create image
   embed for URLs that serve image/* content
4. Audio seeking broken: add HTTP Range request support in uploads
   route (Accept-Ranges, Content-Range, 206 Partial Content)

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

@@ -36,8 +36,8 @@ export function VideoEmbed({ embed }: VideoEmbedProps) {
             className="absolute inset-0 w-full h-full"
             src={`${embed.embedUrl}?autoplay=1`}
             title={embed.title ?? 'Video'}
-            allow="autoplay; fullscreen; picture-in-picture"
-            sandbox="allow-scripts allow-same-origin allow-popups"
+            allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share"
+            allowFullScreen
             referrerPolicy="no-referrer"
           />
         ) : (