fix(web): hide assistant message copy button on mobile (#456) (#458)

The copy button on assistant messages was always visible on mobile
(opacity-60), positioned as a detached row below the message content.
Hide it entirely on small screens (hidden sm:flex) and keep hover-only
behavior on desktop. Mobile users can still copy via native long-press
text selection.

via [HAPI](https://hapi.run)

Co-authored-by: HAPI <noreply@hapi.run>

web/src/components/AssistantChat/messages/AssistantMessage.tsx

@@ -57,7 +57,7 @@ export function HappyAssistantMessage() {
                 <MessagePrimitive.Content components={MESSAGE_PART_COMPONENTS} />
             </div>
             {copyText && (
-                <div className="flex justify-end mt-1 opacity-60 sm:opacity-0 sm:group-hover/msg:opacity-100 transition-opacity">
+                <div className="hidden sm:flex justify-end mt-1 opacity-0 group-hover/msg:opacity-100 transition-opacity">
                     <button
                         type="button"
                         title="Copy"