RevenueCat Capacitor IAP; keep Stripe on web (#100) (#102)

## Summary
- Native Capacitor: RevenueCat purchase/restore (`@revenuecat/purchases-capacitor`)
- Web: Stripe checkout/portal unchanged
- Billing history shows store rows (`provider` / `revenuecat_store`)
- RC `logIn`/`logOut` on auth lifecycle; appUserID = user pk

Closes #100. Depends on backend [chat_backend#68](ai_ml_operations/chat_backend#68) / monetization PR.

## Test plan
- [x] Unit tests: BillingSection, revenueCat, finance helpers, SignIn/Up, AuthCallback, Header2, DeleteAccount (38 OK)
- [ ] Fill `REACT_APP_REVENUECAT_*` keys in `.env.mobile`
- [ ] Align RC offerings/products with plan slugs
- [ ] `npm run build:mobile` + `cap sync`; sandbox purchase on Android/iOS
- [ ] Confirm web Stripe checkout/portal still work
- [ ] After RC webhook, Account → Billing history shows store invoiceReviewed-on: #102

llm-fe/src/setupTests.ts

@@ -3,3 +3,16 @@
 // expect(element).toHaveTextContent(/react/i)
 // learn more: https://github.com/testing-library/jest-dom
 import '@testing-library/jest-dom';
+
+// Capacitor plugin ships ESM; Jest cannot parse it when AccountContext
+// (or pages that import it) pull in utils/revenueCat. Stub globally.
+jest.mock('@revenuecat/purchases-capacitor', () => ({
+  Purchases: {
+    configure: jest.fn(() => Promise.resolve()),
+    logIn: jest.fn(() => Promise.resolve({ customerInfo: {} })),
+    logOut: jest.fn(() => Promise.resolve({ customerInfo: {} })),
+    getOfferings: jest.fn(() => Promise.resolve({ current: null, all: {} })),
+    purchasePackage: jest.fn(() => Promise.resolve({ customerInfo: {} })),
+    restorePurchases: jest.fn(() => Promise.resolve({ customerInfo: {} })),
+  },
+}));