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/shared/container.tsx

@@ -0,0 +1,9 @@
+import { type ComponentProps } from 'react';
+
+import { cn } from '@/lib/utils';
+
+export function Container({ className, ...props }: ComponentProps<'div'>) {
+  return (
+    <div className={cn('mx-auto w-full max-w-6xl px-6', className)} {...props} />
+  );
+}