feat(adminsite): self-hosted purchase, checkout overlay, billing portal, client line-item builder

Purchase flow: name -> placeholder -> configure via the shipped PlanConfigurator
-> Paddle overlay with custom_data -> paste install UUID to link and issue.
lineItemsFor mirrors the Go catalogue.LineItems/billable exactly (base included
in exactly one place). ManageBillingButton opens the hosted portal. Paddle token
and env are baked into the build, never fetched.

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

adminsite/components/ManageBillingButton.tsx

@@ -0,0 +1,35 @@
+"use client";
+
+import { useState } from "react";
+import { ApiError, api } from "@/lib/api";
+import { Button } from "@/components/Button";
+
+/* Opens Paddle's hosted customer portal in a new tab. The account learns its
+ * paddle_customer_id from its first paid subscription's webhook, so this reports
+ * a plain message rather than erroring when there is no billing account yet. */
+export function ManageBillingButton() {
+    const [busy, setBusy] = useState(false);
+    const [note, setNote] = useState<string | null>(null);
+
+    async function open() {
+        setBusy(true);
+        setNote(null);
+        try {
+            const { url } = await api.billingPortal();
+            window.open(url, "_blank", "noopener");
+        } catch (e) {
+            setNote(e instanceof ApiError ? e.message : "Could not open billing.");
+        } finally {
+            setBusy(false);
+        }
+    }
+
+    return (
+        <span className="inline-flex items-center gap-2">
+            <Button type="button" variant="line" onClick={open} disabled={busy}>
+                {busy ? "Opening…" : "Manage billing"}
+            </Button>
+            {note && <span className="text-[0.78rem] text-ink-3">{note}</span>}
+        </span>
+    );
+}