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/App.xcodeproj/project.pbxproj

@@ -283,7 +283,7 @@
 				GCC_WARN_UNINITIALIZED_AUTOS = YES_AGGRESSIVE;
 				GCC_WARN_UNUSED_FUNCTION = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
-				IPHONEOS_DEPLOYMENT_TARGET = 14.0;
+				IPHONEOS_DEPLOYMENT_TARGET = 15.0;
 				MTL_ENABLE_DEBUG_INFO = YES;
 				ONLY_ACTIVE_ARCH = YES;
 				SDKROOT = iphoneos;
@@ -334,7 +334,7 @@
 				GCC_WARN_UNINITIALIZED_AUTOS = YES_AGGRESSIVE;
 				GCC_WARN_UNUSED_FUNCTION = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
-				IPHONEOS_DEPLOYMENT_TARGET = 14.0;
+				IPHONEOS_DEPLOYMENT_TARGET = 15.0;
 				MTL_ENABLE_DEBUG_INFO = NO;
 				SDKROOT = iphoneos;
 				SWIFT_OPTIMIZATION_LEVEL = "-Owholemodule";
@@ -350,7 +350,7 @@
 				CODE_SIGN_STYLE = Automatic;
 				CURRENT_PROJECT_VERSION = 1;
 				INFOPLIST_FILE = App/Info.plist;
-				IPHONEOS_DEPLOYMENT_TARGET = 14.0;
+				IPHONEOS_DEPLOYMENT_TARGET = 15.0;
 				LD_RUNPATH_SEARCH_PATHS = "$(inherited) @executable_path/Frameworks";
 				MARKETING_VERSION = 0.1.0;
 				OTHER_SWIFT_FLAGS = "$(inherited) \"-D\" \"COCOAPODS\" \"-DDEBUG\"";
@@ -370,7 +370,7 @@
 				CODE_SIGN_STYLE = Automatic;
 				CURRENT_PROJECT_VERSION = 1;
 				INFOPLIST_FILE = App/Info.plist;
-				IPHONEOS_DEPLOYMENT_TARGET = 14.0;
+				IPHONEOS_DEPLOYMENT_TARGET = 15.0;
 				LD_RUNPATH_SEARCH_PATHS = "$(inherited) @executable_path/Frameworks";
 				MARKETING_VERSION = 0.1.0;
 				PRODUCT_BUNDLE_IDENTIFIER = ai.hesychia.chat;