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/react-app-env.d.ts

@@ -9,6 +9,7 @@ interface CapacitorPreferencesPlugin {
 interface CapacitorBridge {
   isNativePlatform?: () => boolean;
   isNative?: boolean;
+  getPlatform?: () => string;
   Plugins?: {
     Preferences?: CapacitorPreferencesPlugin;
     App?: { addListener?: (...args: unknown[]) => unknown };