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/.env.mobile

@@ -4,3 +4,9 @@
 #   REACT_APP_BACKEND_WS_API_BASE_URL=wss://beta.chatbackend.aimloperations.com/ws/chat_again/
 REACT_APP_BACKEND_REST_API_BASE_URL=https://chatbackend.aimloperations.com/api/
 REACT_APP_BACKEND_WS_API_BASE_URL=wss://chatbackend.aimloperations.com/ws/chat_again/
+
+# RevenueCat public SDK keys (native IAP). Leave empty until store apps are wired.
+REACT_APP_REVENUECAT_APPLE_API_KEY=
+REACT_APP_REVENUECAT_GOOGLE_API_KEY=
+# Optional: pin a specific offering identifier (defaults to current offering).
+# REACT_APP_REVENUECAT_OFFERING_ID=