Prepare Kalendartool for MPM and disable invites

lib/client-fetch.ts

@@ -0,0 +1,8 @@
+/** Fetch wrapper for absolute app paths when Next.js basePath is enabled. */
+export function appFetch(input: RequestInfo | URL, init?: RequestInit): Promise<Response> {
+  if (typeof input !== 'string' || !input.startsWith('/') || input.startsWith('//')) {
+    return fetch(input, init);
+  }
+  const basePath = process.env.NEXT_PUBLIC_APP_BASE_PATH ?? '';
+  return fetch(`${basePath}${input}`, init);
+}