feat: add 404 page with centered layout

not-found.tsx renders oversized Fraunces heading with a back link.
Body gets flex flex-col min-h-screen so main can flex-1 to fill
available height without pushing the footer off screen.

app/not-found.tsx

@@ -0,0 +1,13 @@
+import { Link } from '$shared/ui';
+
+/**
+ * Custom 404 page — shown for any unmatched route.
+ */
+export default function NotFound() {
+  return (
+    <main className="flex-1 flex flex-col items-center justify-center gap-6">
+      <h1 className="font-heading text-[clamp(8rem,20vw,18rem)] leading-none">404</h1>
+      <Link href="/">Back to main</Link>
+    </main>
+  );
+}