Refine the store assistant UI and gate it behind a flag

- Gate the assistant on NEXT_PUBLIC_ENABLE_AI=1 (UI only; the /api/chat
  route still responds if called directly)
- Chat model is openai/gpt-5.6-luna-pro with reasoning requested and
  sendReasoning enabled, rendered via the Reasoning component
- Suggestion chips and reasoning come from ai-elements; attachments gain
  hover-card previews, left alignment, and render under the user message
- Launcher: plain Button when closed, magicui RainbowButton when open
- Tool results replace the collapsible Tool UI with a shimmer while
  running and a muted icon + summary line after, plus product previews
- Commerce icons in place of the folder icon for collections
- Add a thin announcement bar above the header; footer splits links and
  socials onto separate rows and clears the launcher corner
- Add the shadcn base layer so Tailwind v4's bare `border` picks up the
  theme colour instead of currentColor

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jWbNNJLksC1QG8z8845FX

app/api/chat/route.ts

@@ -13,7 +13,7 @@ import {
 // server-side on each tool call.
 export const maxDuration = 30;
 
-const MODEL = process.env.OPENROUTER_MODEL ?? 'anthropic/claude-sonnet-4.5';
+const MODEL = process.env.OPENROUTER_MODEL ?? 'openai/gpt-5.6-luna-pro';
 
 const SYSTEM_PROMPT = `You are the shopping assistant for an online store built on Shopify.
 
@@ -84,7 +84,9 @@ export async function POST(req: Request) {
   const openrouter = createOpenRouter({ apiKey });
 
   const result = streamText({
-    model: openrouter(MODEL),
+    // `reasoning` asks OpenRouter to stream the model's thinking; the UI
+    // renders it via the Reasoning component.
+    model: openrouter(MODEL, { reasoning: { enabled: true, effort: 'medium' } }),
     system: SYSTEM_PROMPT,
     messages: await convertToModelMessages(messages),
     // Let the model call a tool, read the result, then answer.
@@ -193,5 +195,7 @@ export async function POST(req: Request) {
     },
   });
 
-  return result.toUIMessageStreamResponse();
+  // sendReasoning forwards reasoning parts to the client; without it the
+  // stream carries text and tool calls only.
+  return result.toUIMessageStreamResponse({ sendReasoning: true });
 }