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

README.md

@@ -229,6 +229,7 @@ See [server-infra IMPLEMENTATION.md](https://git.aimloperations.com/ai_ml_operat
 - **Custom events / identify** require analytics consent (`AnalyticsConsentBanner` + `trackEvent` / `identifyUser`)
 - Consent choice stored in `localStorage` key `hesychia_analytics_consent_v1` (`granted` / `denied`)
 - Banner links to `/terms_of_service/#analytics` for the always-on vs optional split
+- Event catalog: [`llm-fe/ANALYTICS.md`](llm-fe/ANALYTICS.md)
 
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