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/components/InstanceForm.tsx

@@ -14,7 +14,7 @@ function slugify(value: string) {
         .replace(/^-|-$/g, "");
 }
 
-export function OrgForm() {
+export function InstanceForm() {
     const [slug, setSlug] = useState("");
     const [result, setResult] = useState<SubmitResult>({ state: "idle" });
     const sending = result.state === "sending";
@@ -25,7 +25,7 @@ export function OrgForm() {
         setResult({ state: "sending" });
         setResult(
             await submitSignup({
-                org_name: String(data.get("org_name") ?? ""),
+                instance_name: String(data.get("instance_name") ?? ""),
                 email: String(data.get("email") ?? ""),
                 password: String(data.get("password") ?? ""),
                 website: String(data.get("website") ?? ""),
@@ -38,7 +38,7 @@ export function OrgForm() {
             <div role="status">
                 <h2 style={{ fontSize: "var(--s-1)" }}>Check your email.</h2>
                 <p style={{ color: "var(--ink-2)", marginTop: "0.5rem" }}>
-                    We sent a confirmation link. Open it and <b>{slug || "your organisation"}</b> is created with you as its owner. The link works once and expires in 24 hours.
+                    We sent a confirmation link. Open it and <b>{slug || "your instance"}</b> is created with you as its owner. The link works once and expires in 24 hours.
                 </p>
                 <p style={{ color: "var(--ink-3)", marginTop: "0.75rem", fontSize: "0.88rem" }}>
                     Nothing exists until you confirm if the email does not arrive, start again or contact support@hostxtra.co.uk.
@@ -54,14 +54,14 @@ export function OrgForm() {
             <Honeypot />
 
             <div className="field">
-                <label htmlFor="o-org">Organisation name</label>
-                <input id="o-org" name="org_name" type="text" placeholder="Northgate Systems" required onChange={(e) => setSlug(slugify(e.target.value))} aria-describedby="o-org-err" />
+                <label htmlFor="o-instance">Instance name</label>
+                <input id="o-instance" name="instance_name" type="text" placeholder="Northgate Systems" required onChange={(e) => setSlug(slugify(e.target.value))} aria-describedby="o-instance-err" />
                 <span className="hostline">
-                    <b>{slug || "your-org"}</b>.vantage.hostxtra.co.uk
+                    <b>{slug || "your-instance"}</b>.vantage.hostxtra.co.uk
                 </span>
-                {fieldError("org_name") && (
-                    <small id="o-org-err" className="field__err">
-                        {fieldError("org_name")}
+                {fieldError("instance_name") && (
+                    <small id="o-instance-err" className="field__err">
+                        {fieldError("instance_name")}
                     </small>
                 )}
             </div>