Buy on StoreKit and Play Billing, then confirm with the backend (#78) (#122)

## Summary
- The Capacitor shell buys through StoreKit (iOS 15) and Play Billing instead of RevenueCat.
- After the store sheet succeeds, the app POSTs the signed transaction or Play purchase token to `/monetization/store/confirm/`.
- Restore reads current store entitlements and confirms each one. Web checkout stays on Stripe.
- The store account token from `GET /monetization/subscription/` is sent as `appAccountToken` / `obfuscatedAccountId`.

Depends on the `chat_backend` PR for issue #78 (`feat/78-native-store-purchases`).

## Test plan
- [x] `storeBilling`, `revenueCat`, `finance`, and `BillingSection` unit tests
- [ ] iOS sandbox purchase and restore against a backend with `APPLE_IAP_BUNDLE_ID` set
- [ ] Play license-test purchase and restore against a backend with `GOOGLE_PLAY_SERVICE_ACCOUNT_JSON` set
- [ ] Web billing still opens Stripe Checkout

Reviewed-on: #122

llm-fe/plugins/native-store-billing/package.json

@@ -0,0 +1,26 @@
+{
+  "name": "native-store-billing",
+  "version": "0.1.0",
+  "private": true,
+  "description": "StoreKit and Play Billing purchases for the Hesychia Capacitor shell.",
+  "main": "dist/index.js",
+  "types": "dist/index.d.ts",
+  "capacitor": {
+    "ios": {
+      "src": "ios"
+    },
+    "android": {
+      "src": "android"
+    }
+  },
+  "files": [
+    "android/src/main/",
+    "android/build.gradle",
+    "ios/Sources",
+    "NativeStoreBilling.podspec",
+    "dist/"
+  ],
+  "peerDependencies": {
+    "@capacitor/core": ">=7.0.0"
+  }
+}