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/ANDROID.md

@@ -40,6 +40,20 @@ npm run android:sync
 
 Default `.env.mobile` matches production (`chatbackend.aimloperations.com`). Point it at beta to flip the shell without touching web deploys. Optional gitignored override: `.env.mobile.local`.
 
+### RevenueCat IAP (#100)
+
+Native billing uses `@revenuecat/purchases-capacitor` (Capacitor 7 → package **11.x**). Web keeps Stripe Checkout / Customer Portal.
+
+Set public SDK keys in `.env.mobile` (empty placeholders OK until store apps ship):
+
+| Variable | Purpose |
+|----------|---------|
+| `REACT_APP_REVENUECAT_APPLE_API_KEY` | iOS public SDK key |
+| `REACT_APP_REVENUECAT_GOOGLE_API_KEY` | Android public SDK key |
+| `REACT_APP_REVENUECAT_OFFERING_ID` | Optional offering pin (else current) |
+
+After install / key changes: `npm run build:mobile` (runs `cap sync`). More detail: [`MONETIZATION.md`](MONETIZATION.md).
+
 ## Versioning
 
 In `android/app/build.gradle`: