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/lib/submit.ts

@@ -56,7 +56,7 @@ export async function submitContact(fields: { name: string; email: string; serve
     return post(`${SITE_API}/api/contact`, fields);
 }
 
-export async function submitSignup(fields: { org_name: string; email: string; password: string; website: string }): Promise<SubmitResult> {
+export async function submitSignup(fields: { instance_name: string; email: string; password: string; website: string }): Promise<SubmitResult> {
     if (!SITE_API) {
         return {
             state: "error",