Tianji custom event tracking (auth, billing, chat) (#36) (#45)

## Summary

Closes #36.

Helpers / consent gating already exist from [#42](#42). This PR adds **call sites** + docs:

- `identifyAccount` + `AnalyticsSession` (identify on session; `Logout` when auth ends)
- Auth: Sign In / Sign Up / AuthCallback (SSO) success & fail
- Billing: Checkout started, Payment Success / Canceled
- Chat: Conversation Created, Message Sent (metadata only — no prompt text)
- ToS Acknowledged
- `BILLING_PORTAL_OPENED` reserved until Account portal (#33)
- Catalog: [`llm-fe/ANALYTICS.md`](llm-fe/ANALYTICS.md)

No `chat_backend` changes required.

## Test plan

- [ ] Grant consent on beta → sign in → Tianji shows Login Success + identify
- [ ] Decline consent → same actions produce no custom events (page views still ok)
- [ ] Sign up + checkout + cancel/success return URLs fire expected events
- [ ] New chat + send message → Conversation Created / Message Sent without prompt text
- [ ] Acknowledge ToS → ToS Acknowledged
- [ ] `npm run test:ci -- --testPathPattern='analytics.test|SignIn.test|SignUp.test|AuthCallback.test'`Reviewed-on: #45

llm-fe/src/App.tsx

@@ -22,6 +22,7 @@ import GlobalThemeWrapper from './llm-fe/components/GlobalThemeWrapper/GlobalThe
 import Tracker from './llm-fe/components/Tracker/Tracker';
 import { AnalyticsConsentProvider } from './llm-fe/contexts/AnalyticsConsentContext';
 import AnalyticsConsentBanner from './llm-fe/components/AnalyticsConsentBanner/AnalyticsConsentBanner';
+import AnalyticsSession from './llm-fe/components/AnalyticsSession/AnalyticsSession';
 
 const ProtectedRoutes = () => {
   const { authenticated, loading } = useContext(AuthContext);
@@ -45,6 +46,7 @@ class App extends Component {
       <GlobalThemeWrapper>
         <AnalyticsConsentProvider>
         <Tracker />
+        <AnalyticsSession />
         <AnalyticsConsentBanner />
         <div className='site'>
           <main>