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/android/app/capacitor.build.gradle

@@ -13,6 +13,7 @@ dependencies {
     implementation project(':capacitor-keyboard')
     implementation project(':capacitor-preferences')
     implementation project(':capacitor-status-bar')
+    implementation project(':native-store-billing')
 
 }
 

llm-fe/android/capacitor.settings.gradle

@@ -13,3 +13,6 @@ project(':capacitor-preferences').projectDir = new File('../node_modules/@capaci
 
 include ':capacitor-status-bar'
 project(':capacitor-status-bar').projectDir = new File('../node_modules/@capacitor/status-bar/android')
+
+include ':native-store-billing'
+project(':native-store-billing').projectDir = new File('../plugins/native-store-billing/android')