Add theme toggle, hero snippet, and reorganize homepage components

- Set up next-themes with a light/dark toggle in the header and move all
  sections onto theme tokens; dark mode stays pure black
- Replace the hero terminal line with the AI Elements Snippet component
- Restructure components into layout/, sections/, and shared/ with
  site-wide config in lib/site.ts

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

components.json

@@ -22,6 +22,7 @@
   "menuColor": "default",
   "menuAccent": "subtle",
   "registries": {
-    "@magicui": "https://magicui.design/r/{name}"
+    "@magicui": "https://magicui.design/r/{name}",
+    "@ai-elements": "https://ai-sdk.dev/elements/api/registry/{name}.json"
   }
 }