fix: exclude embed thumbnails from image context menu actions

Add data-embed-thumbnail marker to VideoEmbed, GenericEmbed, and
RichEmbed thumbnail containers. Update detection logic to skip
these alongside avatars. Also remove redundant filename derivation
in Save Image menu item.

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

@@ -41,7 +41,7 @@ export function GenericEmbed({ embed }: GenericEmbedProps) {
         )}
       </div>
       {embed.image && (
-        <div className="w-[80px] h-[80px] m-3 flex-shrink-0">
+        <div data-embed-thumbnail className="w-[80px] h-[80px] m-3 flex-shrink-0">
           <img
             src={embed.image}
             alt=""

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

@@ -50,7 +50,7 @@ export function RichEmbed({ embed }: RichEmbedProps) {
         >
           <div className="flex items-start gap-3 p-3">
             {embed.image && (
-              <div className="w-[80px] h-[80px] flex-shrink-0">
+              <div data-embed-thumbnail className="w-[80px] h-[80px] flex-shrink-0">
                 <img
                   src={embed.image}
                   alt=""

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

@@ -45,6 +45,7 @@ export function VideoEmbed({ embed }: VideoEmbedProps) {
         ) : (
           <button
             type="button"
+            data-embed-thumbnail
             onClick={() => setIsPlaying(true)}
             className="absolute inset-0 w-full h-full flex items-center justify-center group focus:outline-none"
             aria-label={`Play ${embed.title ?? 'video'}`}