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/globals.css

@@ -1,5 +1,7 @@
 @import 'tailwindcss';
 
+@custom-variant dark (&:is(.dark *));
+
 @theme {
   /* Refined neutral modern palette */
   --color-background: hsl(0 0% 100%);
@@ -56,6 +58,15 @@
 }
 
 /* Base styles for light, modern Shopify storefront */
+/* Tailwind v4 defaults an unqualified `border` to currentColor. shadcn
+   components (e.g. Button's outline variant) rely on this base layer to pick up
+   the theme's border colour instead of the text colour. */
+@layer base {
+  * {
+    border-color: var(--color-border);
+  }
+}
+
 body {
   font-feature-settings:
     'kern' 1,
@@ -184,3 +195,36 @@ button,
 .price-display {
   font-feature-settings: 'tnum';
 }
+
+@theme inline {
+  --animate-rainbow: rainbow var(--speed, 2s) infinite linear;
+  --color-color-5: var(--color-5);
+  --color-color-4: var(--color-4);
+  --color-color-3: var(--color-3);
+  --color-color-2: var(--color-2);
+  --color-color-1: var(--color-1);
+  @keyframes rainbow {
+  0% {
+    background-position: 0%;
+    }
+  100% {
+    background-position: 200%;
+    }
+  }
+}
+
+:root {
+  --color-1: oklch(66.2% 0.225 25.9);
+  --color-2: oklch(60.4% 0.26 302);
+  --color-3: oklch(69.6% 0.165 251);
+  --color-4: oklch(80.2% 0.134 225);
+  --color-5: oklch(90.7% 0.231 133);
+}
+
+.dark {
+  --color-1: oklch(66.2% 0.225 25.9);
+  --color-2: oklch(60.4% 0.26 302);
+  --color-3: oklch(69.6% 0.165 251);
+  --color-4: oklch(80.2% 0.134 225);
+  --color-5: oklch(90.7% 0.231 133);
+}