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/package-lock.json

@@ -21,6 +21,7 @@
         "@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",
@@ -5239,6 +5240,24 @@
         "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",