refactor(site): rename Organisation to Instance

The signup form now posts instance_name, matching sitesvc.

Also restores site/next-env.d.ts. Its /// <reference> directives had been
stripped, which removed the Next.js type environment and failed the build
with "Cannot find name 'Promise'". Same cause as the agent's missing build
constraint.

site/next-env.d.ts

@@ -1,6 +1,6 @@
-/
-/
-import "./.next/dev/types/routes.d.ts";
-
-
+/// <reference types="next" />
+/// <reference types="next/image-types/global" />
+import "./.next/types/routes.d.ts";
 
+// NOTE: This file should not be edited
+// see https://nextjs.org/docs/app/api-reference/config/typescript for more information.