fix(frontend): make Suggestion button opaque in dark mode (#2276)

* fix(frontend): make Suggestion button opaque in dark mode

The outline Button variant applies dark:bg-input/30, leaving Suggestion
pills ~70% transparent in dark mode. Scrolled chat content bled through
the buttons, making suggestion text unreadable. Override with
dark:bg-background so it matches the opaque light-mode appearance.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix the lint error of commit

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/src/components/ai-elements/suggestion.tsx

@@ -64,7 +64,7 @@ export const Suggestion = ({
   return (
     <Button
       className={cn(
-        "text-muted-foreground h-auto max-w-full cursor-pointer rounded-full px-4 py-2 text-center text-xs font-normal whitespace-normal",
+        "text-muted-foreground dark:bg-background h-auto max-w-full cursor-pointer rounded-full px-4 py-2 text-center text-xs font-normal whitespace-normal",
         className,
       )}
       onClick={handleClick}