fix: user bubble color not rendering — use Tailwind v4 variable syntax

The bg-[var(--user-bubble)] arbitrary value syntax wasn't working with
Tailwind v4's Lightning CSS pipeline. Switch to bg-(--user-bubble) which
is Tailwind v4's native CSS variable reference syntax.

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

src/components/ai-elements/message.tsx

@@ -56,7 +56,7 @@ export const MessageContent = ({
   <div
     className={cn(
       "is-user:dark flex w-fit min-w-0 max-w-full flex-col gap-2 overflow-hidden text-sm",
-      "group-[.is-user]:ml-auto group-[.is-user]:rounded-lg group-[.is-user]:bg-[var(--user-bubble)] group-[.is-user]:px-4 group-[.is-user]:py-3 group-[.is-user]:text-[var(--user-bubble-foreground)]",
+      "group-[.is-user]:ml-auto group-[.is-user]:rounded-lg group-[.is-user]:bg-(--user-bubble) group-[.is-user]:px-4 group-[.is-user]:py-3 group-[.is-user]:text-(--user-bubble-foreground)",
       "group-[.is-assistant]:w-full group-[.is-assistant]:text-foreground",
       className
     )}