feat: add permission-gated navigation and admin access

src/app/not-found.tsx

@@ -0,0 +1,21 @@
+import Link from 'next/link';
+
+import { Button } from '@/components/ui/button';
+import { ROUTES } from '@/utils/routes';
+
+export default function NotFound() {
+  return (
+    <main className="flex min-h-screen items-center justify-center bg-background p-6">
+      <div className="max-w-md text-center">
+        <p className="text-sm font-medium text-muted-foreground">404</p>
+        <h1 className="mt-2 text-3xl font-semibold">Page not found</h1>
+        <p className="mt-3 text-sm text-muted-foreground">
+          The page you are looking for does not exist or may have been moved.
+        </p>
+        <Button asChild className="mt-6">
+          <Link href={ROUTES.DASHBOARD}>Go to dashboard</Link>
+        </Button>
+      </div>
+    </main>
+  );
+}