Wire Tianji custom event tracking for auth, billing, and chat (#36)

Add identifyAccount + AnalyticsSession, instrument MVP funnel call sites without PII payloads, and document event names in ANALYTICS.md. Billing portal event reserved until #33.

llm-fe/ANALYTICS.md

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+# Tianji product events (Hesychia)
+
+Helpers: `llm-fe/src/llm-fe/utils/analytics.ts`  
+Consent gate: custom events + identify only when `hesychia_analytics_consent*` is `granted` (see #37).  
+Page views: always via `Tracker` / `tracker.js` on prod + beta.
+
+## Event catalog
+
+| Event name | Constant | Call site(s) | Payload (non-PII) |
+|------------|----------|--------------|-------------------|
+| Login Success | `LOGIN_SUCCESS` | SignIn, AuthCallback | `{ method: 'password' \| 'sso' }` |
+| Login Failed | `LOGIN_FAILED` | SignIn, AuthCallback | `{ method }` |
+| Sign Up Started | `SIGNUP_STARTED` | SignUp | — |
+| Sign Up Success | `SIGNUP_SUCCESS` | SignUp, AuthCallback (`created`) | `{ method? }` |
+| Sign Up Failed | `SIGNUP_FAILED` | SignUp, AuthCallback | `{ method? }` |
+| Logout | `LOGOUT` | AnalyticsSession (auth → logged out) | — |
+| Checkout Started | `CHECKOUT_STARTED` | SignUp, AuthCallback | `{ source }` |
+| Payment Success | `PAYMENT_SUCCESS` | BillingSuccess | `{ hasSessionId }` |
+| Payment Canceled | `PAYMENT_CANCELED` | BillingCancel | — |
+| Conversation Created | `CONVERSATION_CREATED` | MessageContext | `{ conversationId }` |
+| Message Sent | `MESSAGE_SENT` | AsyncDashboard2 | `{ hasConversation, hasAttachment }` |
+| ToS Acknowledged | `TOS_ACKNOWLEDGED` | TermsOfService | — |
+| Billing Portal Opened | `BILLING_PORTAL_OPENED` | *(pending #33 Account portal CTA)* | — |
+
+## Identify
+
+`identifyAccount(account)` sends `{ userId: email, companyId?, isCompanyManager? }`.  
+No message text, document contents, or names.
+
+## Environments
+
+Only prod/beta (`REACT_APP_DEPLOY_ENV` or production build with Tianji website id). Local dev skips unless env set.