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-lock.json

@@ -21,7 +21,6 @@
         "@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",
@@ -39,6 +38,7 @@
         "lodash": "^4.17.21",
         "markdown-to-jsx": "^7.7.2",
         "mini.css": "^3.0.1",
+        "native-store-billing": "file:plugins/native-store-billing",
         "papaparse": "^5.5.4",
         "pdfmake": "^0.3.11",
         "react-bootstrap": "^2.10.6",
@@ -5239,24 +5239,6 @@
         "react": ">=16.14.0"
       }
     },
-    "node_modules/@revenuecat/purchases-capacitor": {
-      "version": "11.3.2",
-      "resolved": "https://registry.npmjs.org/@revenuecat/purchases-capacitor/-/purchases-capacitor-11.3.2.tgz",
-      "integrity": "sha512-3T4/lcAwpbPagrT4DuXvJ+8RewzFmHf3fQJQuhVy+uTQk5HGMHNlXcm2A/UdyHtWqU+/VqWbCClPFYGnpnNXAQ==",
-      "license": "MIT",
-      "dependencies": {
-        "@revenuecat/purchases-typescript-internal-esm": "17.25.0"
-      },
-      "peerDependencies": {
-        "@capacitor/core": ">=7.0.0"
-      }
-    },
-    "node_modules/@revenuecat/purchases-typescript-internal-esm": {
-      "version": "17.25.0",
-      "resolved": "https://registry.npmjs.org/@revenuecat/purchases-typescript-internal-esm/-/purchases-typescript-internal-esm-17.25.0.tgz",
-      "integrity": "sha512-KC4BjFaQclXqFafG1Enh7t8GSwdg8p905by7UrHq0WSJmLhOR6yXSj6WDv1iwsFSAZxPPwgozNxt+U6IOFTdCA==",
-      "license": "MIT"
-    },
     "node_modules/@rollup/plugin-babel": {
       "version": "5.3.1",
       "resolved": "https://registry.npmjs.org/@rollup/plugin-babel/-/plugin-babel-5.3.1.tgz",
@@ -16639,6 +16621,10 @@
         "node": ">= 10.x"
       }
     },
+    "node_modules/native-store-billing": {
+      "resolved": "plugins/native-store-billing",
+      "link": true
+    },
     "node_modules/natural-compare": {
       "version": "1.4.0",
       "resolved": "https://registry.npmjs.org/natural-compare/-/natural-compare-1.4.0.tgz",
@@ -23984,6 +23970,12 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "plugins/native-store-billing": {
+      "version": "0.1.0",
+      "peerDependencies": {
+        "@capacitor/core": ">=7.0.0"
+      }
+    },
     "vendor/brace-expansion-compat": {
       "name": "brace-expansion",
       "version": "5.0.8",