## 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
westfarn