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

package.json

@@ -12,6 +12,7 @@
         "@ai-sdk/react": "^4.0.47",
         "@next/swc-wasm-web": "16.2.10",
         "@openrouter/ai-sdk-provider": "^3.0.0",
+        "@radix-ui/react-use-controllable-state": "^1.2.6",
         "@remixicon/react": "^4.9.0",
         "@shopify/storefront-api-client": "^1.0.0",
         "@streamdown/cjk": "^1.0.3",