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/section-heading.tsx

@@ -0,0 +1,44 @@
+import { cn } from '@/lib/utils';
+
+export function SectionHeading({
+  eyebrow,
+  title,
+  description,
+  align = 'left',
+  className,
+}: {
+  eyebrow?: string;
+  title: string;
+  description?: string;
+  align?: 'left' | 'center';
+  className?: string;
+}) {
+  return (
+    <div
+      className={cn(
+        'max-w-2xl',
+        align === 'center' && 'mx-auto text-center',
+        className,
+      )}
+    >
+      {eyebrow && (
+        <p className="font-mono text-xs uppercase tracking-[0.2em] text-foreground/40">
+          {eyebrow}
+        </p>
+      )}
+      <h2
+        className={cn(
+          'text-3xl font-semibold tracking-tight sm:text-4xl',
+          eyebrow && 'mt-4',
+        )}
+      >
+        {title}
+      </h2>
+      {description && (
+        <p className="mt-5 text-lg leading-8 text-foreground/55">
+          {description}
+        </p>
+      )}
+    </div>
+  );
+}