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/gradient-glow.tsx

@@ -0,0 +1,21 @@
+import { cn } from '@/lib/utils';
+
+/**
+ * Soft conic-gradient bloom used behind hero and CTA copy.
+ * Position and size it with `className`; it never captures pointer events.
+ */
+export function GradientGlow({ className }: { className?: string }) {
+  return (
+    <div
+      aria-hidden="true"
+      className={cn(
+        'pointer-events-none absolute rounded-full blur-3xl',
+        className,
+      )}
+      style={{
+        background:
+          'conic-gradient(from 180deg at 50% 50%, #7c3aed, #2563eb, #06b6d4, #ec4899, #7c3aed)',
+      }}
+    />
+  );
+}