feat(site): /start creates an account, not an instance

The form posts to admin's signup and the slug preview goes: there is no
instance at this point, and previewing one promises something the
submission does not create. Creating the instance is now a step in the
portal, which the page's What happens next panel spells out.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

site/lib/submit.ts

@@ -9,6 +9,7 @@
  */
 
 const SITE_API = (process.env.NEXT_PUBLIC_SITE_API ?? "").replace(/\/$/, "");
+const ADMIN_API = (process.env.NEXT_PUBLIC_ADMIN_API_URL ?? "").replace(/\/$/, "");
 const FALLBACK_ADDRESS = process.env.NEXT_PUBLIC_CONTACT_EMAIL ?? "support@hostxtra.co.uk";
 
 export type SubmitState = "idle" | "sending" | "sent" | "error";
@@ -56,12 +57,23 @@ export async function submitContact(fields: { name: string; email: string; serve
     return post(`${SITE_API}/api/contact`, fields);
 }
 
-export async function submitSignup(fields: { instance_name: string; email: string; password: string; website: string }): Promise<SubmitResult> {
-    if (!SITE_API) {
+/*
+ * Account signup posts to the admin service, not sitesvc. The two form targets
+ * are deliberately separate variables rather than one base URL: contact and
+ * signup are owned by different services, and an implied shared host is how they
+ * silently end up pointing at the wrong one.
+ */
+export async function submitAccountSignup(fields: {
+    name: string;
+    email: string;
+    password: string;
+    website: string;
+}): Promise<SubmitResult> {
+    if (!ADMIN_API) {
         return {
             state: "error",
-            message: `Signup is not available from here yet. Email ${FALLBACK_ADDRESS} and we will set you up.`,
+            message: `Signup is not connected yet. Email ${FALLBACK_ADDRESS} and we will set you up.`,
         };
     }
-    return post(`${SITE_API}/api/signup`, fields);
+    return post(`${ADMIN_API}/auth/signup`, fields);
 }