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/action-buttons.tsx

@@ -0,0 +1,41 @@
+import { Button } from '@/components/ui/button';
+import { cn } from '@/lib/utils';
+
+type Action = { label: string; href: string };
+
+export function ActionButtons({
+  primary,
+  secondary,
+  className,
+}: {
+  primary: Action;
+  secondary?: Action;
+  className?: string;
+}) {
+  return (
+    <div
+      className={cn(
+        'flex flex-col items-center justify-center gap-3 sm:flex-row',
+        className,
+      )}
+    >
+      <Button
+        asChild
+        size="lg"
+        className="h-11 rounded-full bg-foreground px-6 text-[15px] text-background hover:bg-foreground/90"
+      >
+        <a href={primary.href}>{primary.label}</a>
+      </Button>
+      {secondary && (
+        <Button
+          asChild
+          size="lg"
+          variant="outline"
+          className="h-11 rounded-full border-foreground/20 bg-transparent px-6 text-[15px] text-foreground hover:bg-foreground/5 hover:text-foreground"
+        >
+          <a href={secondary.href}>{secondary.label}</a>
+        </Button>
+      )}
+    </div>
+  );
+}