Support basePath in all queries.

pages/logout.js

@@ -1,8 +1,12 @@
 import { useEffect } from 'react';
+import { useRouter } from 'next/router';
 
 export default function LogoutPage() {
+  const router = useRouter();
+  const { basePath } = router;
+
   useEffect(() => {
-    fetch('/api/auth/logout').then(() => (window.location.href = '/login'));
+    fetch(`${basePath}/api/auth/logout`).then(() => router.push('/login'));
   }, []);
 
   return null;