fix(org): route admin and tutor invites to app domain (#731)

Team member invite emails and link-invite copy URLs were built with the
org tenant host (custom domain or siteName.myclassroomio.com). Admin and
tutor invites should always point at the admin dashboard origin
(app.classroomio.com in cloud). Student org invites are unchanged.

packages/core/src/config/dashboard-url.ts

@@ -46,3 +46,11 @@ export function getDashboardBaseUrl(org?: DashboardOrg): string {
 
   return `https://app.${BRAND_ROOT_DOMAIN}`;
 }
+
+/**
+ * Returns the admin dashboard origin (`app.classroomio.com` in cloud).
+ * Use for team-member invite links (admin/tutor) — not student-facing URLs.
+ */
+export function getAppBaseUrl(): string {
+  return getDashboardBaseUrl();
+}