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/start/page.tsx

@@ -1,9 +1,9 @@
 import type { Metadata } from "next";
-import { OrgForm } from "@/components/OrgForm";
+import { InstanceForm } from "@/components/InstanceForm";
 
 export const metadata: Metadata = {
     title: "Vantage Cloud",
-    description: "An organisation owns its servers, keys, workflows, monitors and secrets. Free for three servers, hosted or self-hosted.",
+    description: "An instance owns its servers, keys, workflows, monitors and secrets. Free for three servers, hosted or self-hosted.",
 };
 
 export default function StartPage() {
@@ -12,14 +12,14 @@ export default function StartPage() {
             <div className="split">
                 <div>
                     <span className="tag">Vantage Cloud</span>
-                    <h1 style={{ fontSize: "var(--s-3)", margin: "0.8rem 0 1rem", maxWidth: "15ch" }}>Set up your organisation.</h1>
+                    <h1 style={{ fontSize: "var(--s-3)", margin: "0.8rem 0 1rem", maxWidth: "15ch" }}>Set up your instance.</h1>
                     <p className="lede" style={{ fontSize: "var(--s-0)" }}>
-                        An organisation owns its servers, keys, workflows, monitors and secrets. Nothing inside it is visible to any other organisation. Confirm your email and it is created with you
+                        An instance owns its servers, keys, workflows, monitors and secrets. Nothing inside it is visible to any other instance. Confirm your email and it is created with you
                         as its owner.
                     </p>
 
                     <div className="card" style={{ marginTop: "1.9rem" }}>
-                        <OrgForm />
+                        <InstanceForm />
                     </div>
                 </div>
 
@@ -30,7 +30,7 @@ export default function StartPage() {
                             <span className="spec__k">FIRST</span>
                             <div>
                                 <h3>Confirm your email</h3>
-                                <p>We send a link that works once. Your organisation is created when you open it, not before.</p>
+                                <p>We send a link that works once. Your instance is created when you open it, not before.</p>
                             </div>
                         </div>
                         <div className="spec">