Buy on StoreKit and Play Billing, then confirm with the backend (#78).

Native purchases replace RevenueCat. The app sends the signed transaction or Play token to /monetization/store/confirm/ so access is granted only after the store check.

llm-fe/plugins/native-store-billing/dist/index.d.ts

@@ -0,0 +1 @@
+export {};

llm-fe/plugins/native-store-billing/dist/index.js

@@ -0,0 +1,2 @@
+// Native code is loaded by Capacitor. The web app registers the plugin from src.
+module.exports = {};