fix: replace broken border-hard/50 opacity with subtle white borders

Tailwind can't decompose CSS variables for opacity modifiers, so
border-border-hard/50 fell back to white currentColor. Replaced with
border-white/[0.06] on image attachments, code blocks, and action
toolbar. Also applied frosted glass material to message action toolbar.

packages/web/src/components/chat/MessageList.tsx

@@ -113,7 +113,7 @@ export function MessageList({ channelId }: MessageListProps) {
   return (
     <div
       ref={containerRef}
-      className="flex-1 overflow-y-auto overflow-x-hidden scrollbar-thin"
+      className="flex-1 overflow-y-auto overflow-x-hidden no-scrollbar"
       onScroll={handleScroll}
     >
       {isLoadingMore && (