Set up Storybook again

src/@types/mdx.d.ts

@@ -0,0 +1,12 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { JSX as ReactJSX } from "react";
+
+declare module "mdx/types.js" {
+  export import JSX = ReactJSX;
+}