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/app/pricing/page.tsx

@@ -45,7 +45,7 @@ export default function PricingPage() {
           <ul>
             <li>Up to 3 servers</li>
             <li>Keys, workflows and monitors</li>
-            <li>One member, one organisation</li>
+            <li>One member, one instance</li>
             <li>Community support</li>
           </ul>
           <Link className="btn btn--line" href="/start">