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.md

@@ -40,19 +40,13 @@ npm run android:sync
 
 Default `.env.mobile` matches production (`chatbackend.aimloperations.com`). Point it at beta to flip the shell without touching web deploys. Optional gitignored override: `.env.mobile.local`.
 
-### RevenueCat IAP (#100)
+### Play Billing (#78)
 
-Native billing uses `@revenuecat/purchases-capacitor` (Capacitor 7 → package **11.x**). Web keeps Stripe Checkout / Customer Portal.
+Native billing uses the local Capacitor plugin `native-store-billing` (Play Billing on Android, StoreKit on iOS). Web keeps Stripe Checkout / Customer Portal.
 
-Set public SDK keys in `.env.mobile` (empty placeholders OK until store apps ship):
+The Play product id is the plan's `store_product_id`. The backend verifies the purchase token with the Play Developer API (`GOOGLE_PLAY_SERVICE_ACCOUNT_JSON`). More detail: [`MONETIZATION.md`](MONETIZATION.md).
 
-| Variable | Purpose |
-|----------|---------|
-| `REACT_APP_REVENUECAT_APPLE_API_KEY` | iOS public SDK key |
-| `REACT_APP_REVENUECAT_GOOGLE_API_KEY` | Android public SDK key |
-| `REACT_APP_REVENUECAT_OFFERING_ID` | Optional offering pin (else current) |
-
-After install / key changes: `npm run build:mobile` (runs `cap sync`). More detail: [`MONETIZATION.md`](MONETIZATION.md).
+After plugin changes: `npm run build:mobile` (runs `cap sync`).
 
 ## Versioning