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/lib/paddle.ts

@@ -0,0 +1,17 @@
+import { initializePaddle, type Paddle } from "@paddle/paddle-js";
+
+let cached: Promise<Paddle | undefined> | null = null;
+
+/* One Paddle instance for the app. The token and environment are baked into the
+ * build (NEXT_PUBLIC_*), never fetched, so a production build can never load a
+ * sandbox token by accident. */
+export function initPaddle(): Promise<Paddle | undefined> {
+    if (!cached) {
+        cached = initializePaddle({
+            environment:
+                (process.env.NEXT_PUBLIC_PADDLE_ENV as "sandbox" | "production") ?? "sandbox",
+            token: process.env.NEXT_PUBLIC_PADDLE_CLIENT_TOKEN ?? "",
+        });
+    }
+    return cached;
+}