feat: BLACK HEART DANCE HOUSE landing page

Landing page with Hero, About, Team, Classes, and Contact sections.
Light/dark mode, scroll reveal animations, Yandex Maps, responsive design.
Next.js 16 + Tailwind v4 + TypeScript.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/not-found.tsx

@@ -0,0 +1,13 @@
+import { Button } from "@/components/ui/Button";
+
+export default function NotFound() {
+  return (
+    <div className="flex min-h-[60vh] flex-col items-center justify-center px-4 text-center">
+      <h1 className="font-display text-6xl font-bold">404</h1>
+      <p className="body-text mt-4 text-lg">Страница не найдена</p>
+      <div className="mt-8">
+        <Button href="/">На главную</Button>
+      </div>
+    </div>
+  );
+}