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/brand.tsx

@@ -0,0 +1,20 @@
+import { cn } from '@/lib/utils';
+import { siteConfig } from '@/lib/site';
+
+export function Brand({ className }: { className?: string }) {
+  return (
+    <span className={cn('flex items-center gap-2 font-medium', className)}>
+      <svg
+        width="22"
+        height="22"
+        viewBox="0 0 24 24"
+        fill="none"
+        aria-hidden="true"
+        className="shrink-0"
+      >
+        <path d="M12 2L23 21H1L12 2Z" fill="currentColor" />
+      </svg>
+      <span className="text-[17px] tracking-tight">{siteConfig.name}</span>
+    </span>
+  );
+}