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/package.json

@@ -11,12 +11,12 @@
     "@capacitor/keyboard": "^7.0.6",
     "@capacitor/preferences": "^7.0.4",
     "@capacitor/status-bar": "^7.0.6",
+    "native-store-billing": "file:plugins/native-store-billing",
     "@emotion/cache": "^11.14.0",
     "@emotion/react": "^11.14.0",
     "@emotion/styled": "^11.14.0",
     "@mui/icons-material": "^5.16.11",
     "@mui/material": "^5.16.11",
-    "@revenuecat/purchases-capacitor": "^11.3.2",
     "@testing-library/jest-dom": "^6.6.3",
     "@testing-library/react": "^16.1.0",
     "@testing-library/user-event": "^14.5.2",