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>

app/globals.css

@@ -118,7 +118,7 @@
 }
 
 .dark {
-  --background: oklch(0.145 0 0);
+  --background: oklch(0 0 0);
   --foreground: oklch(0.985 0 0);
   --card: oklch(0.205 0 0);
   --card-foreground: oklch(0.985 0 0);