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/package.json

@@ -9,11 +9,12 @@
         "lint": "next lint"
     },
     "dependencies": {
+        "@paddle/paddle-js": "^1.6.4",
+        "@tanstack/react-query": "^5.51.1",
+        "clsx": "^2.1.1",
         "next": "16.2.9",
         "react": "^18.3.1",
-        "react-dom": "^18.3.1",
-        "@tanstack/react-query": "^5.51.1",
-        "clsx": "^2.1.1"
+        "react-dom": "^18.3.1"
     },
     "devDependencies": {
         "@types/node": "^20.14.11",