fix(org-site): 404 on subdomains that don't resolve to an org

A subdomain like xxazzz.myclassroomio.com set isOrgSite=true with a null
org, and the course page fell back to resolving the org from the course
itself — so courses rendered under subdomains that don't exist. Guard
the (org-site) layout so an unresolved org site 404s instead.

apps/dashboard/src/routes/(org-site)/+layout.server.ts

@@ -0,0 +1,13 @@
+import { error } from '@sveltejs/kit';
+
+export const load = async ({ parent }) => {
+  const { org, isOrgSite } = await parent();
+
+  // An org-site subdomain that doesn't resolve to a real org must 404 rather
+  // than fall back to whatever org a requested course happens to belong to.
+  if (isOrgSite && !org) {
+    throw error(404, 'Organization not found');
+  }
+
+  return {};
+};