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

components.json

@@ -10,6 +10,7 @@
     "cssVariables": true,
     "prefix": ""
   },
+  "iconLibrary": "lucide",
   "aliases": {
     "components": "@/components",
     "utils": "@/lib/utils",
@@ -17,5 +18,7 @@
     "lib": "@/lib",
     "hooks": "@/hooks"
   },
-  "iconLibrary": "lucide"
+  "registries": {
+    "@magicui": "https://magicui.design/r/{name}"
+  }
 }