fix: 404.mdx → 404.tsx

Author: atsuki44

core/morph/include/starter_template/src/pages/404.mdx

@@ -0,0 +1,8 @@
+export default function NotFound() {
+  return (
+    <>
+      <h1>404</h1>
+      <p>Page not found</p>
+    </>
+  );
+}