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/ios/App/Podfile

@@ -1,6 +1,6 @@
 require_relative '../../node_modules/@capacitor/ios/scripts/pods_helpers'
 
-platform :ios, '14.0'
+platform :ios, '15.0'
 use_frameworks!
 
 # workaround to avoid Xcode caching of Pods that requires
@@ -15,6 +15,7 @@ def capacitor_pods
   pod 'CapacitorKeyboard', :path => '../../node_modules/@capacitor/keyboard'
   pod 'CapacitorPreferences', :path => '../../node_modules/@capacitor/preferences'
   pod 'CapacitorStatusBar', :path => '../../node_modules/@capacitor/status-bar'
+  pod 'NativeStoreBilling', :path => '../../plugins/native-store-billing'
 end
 
 target 'App' do