feat: Move manage billing button to billing page

adminsite/app/(customer)/billing/page.tsx

@@ -5,6 +5,7 @@ import { API_BASE, NotConnected, api } from "@/lib/api";
 import { NotConnectedPanel } from "@/components/NotConnected";
 import { PageFrame, RailCard, RailFacts } from "@/components/PageFrame";
 import { PageHeader } from "@/components/PageHeader";
+import { ManageBillingButton } from "@/components/ManageBillingButton";
 import { formatDate } from "@/lib/format";
 
 export default function BillingPage() {
@@ -51,9 +52,12 @@ export default function BillingPage() {
                         </RailCard>
                         <RailCard title="Need a change?">
                             <p className="text-[0.82rem] text-ink-2">
-                                To change a card, download an invoice or cancel, email support and
-                                we will send you a billing link. Self-service billing arrives with
-                                card payments.
+                                Change a card, download an invoice or cancel from the billing
+                                portal. It covers every subscription on this account.
+                            </p>
+                            <ManageBillingButton />
+                            <p className="text-[0.82rem] text-ink-2">
+                                Anything else, email support.
                             </p>
                             <a
                                 href="mailto:support@hostxtra.co.uk"