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-lock.json

@@ -8,6 +8,7 @@
             "name": "vantage-adminsite",
             "version": "0.1.0",
             "dependencies": {
+                "@paddle/paddle-js": "^1.6.4",
                 "@tanstack/react-query": "^5.51.1",
                 "clsx": "^2.1.1",
                 "next": "16.2.9",
@@ -1309,6 +1310,12 @@
                 "node": ">=12.4.0"
             }
         },
+        "node_modules/@paddle/paddle-js": {
+            "version": "1.6.4",
+            "resolved": "https://registry.npmjs.org/@paddle/paddle-js/-/paddle-js-1.6.4.tgz",
+            "integrity": "sha512-ncfnS6I8mCX6krZ3Sgz2iAYivGmhdI81yt9mT6prtPj4Ipd9J3M12LCJRUFL4FB7BYeeuV04c33RSEnbZUBCaA==",
+            "license": "Apache-2.0"
+        },
         "node_modules/@rtsao/scc": {
             "version": "1.1.0",
             "resolved": "https://registry.npmjs.org/@rtsao/scc/-/scc-1.1.0.tgz",