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

yarn.lock

@@ -1710,7 +1710,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"@radix-ui/react-use-controllable-state@npm:1.2.6":
+"@radix-ui/react-use-controllable-state@npm:1.2.6, @radix-ui/react-use-controllable-state@npm:^1.2.6":
   version: 1.2.6
   resolution: "@radix-ui/react-use-controllable-state@npm:1.2.6"
   dependencies:
@@ -4802,6 +4802,7 @@ __metadata:
     "@ai-sdk/react": "npm:^4.0.47"
     "@next/swc-wasm-web": "npm:16.2.10"
     "@openrouter/ai-sdk-provider": "npm:^3.0.0"
+    "@radix-ui/react-use-controllable-state": "npm:^1.2.6"
     "@remixicon/react": "npm:^4.9.0"
     "@shopify/storefront-api-client": "npm:^1.0.0"
     "@streamdown/cjk": "npm:^1.0.3"