feat: add PostHog product analytics for key user actions (#110)

Tracks 9 key events: account_created, login_success, flow_viewed,
session_started, session_completed, export_generated, ai_feature_used,
psa_connected, session_shared. Identifies users on login, resets on
logout. Autocapture disabled — only explicit discrete events.

Set VITE_POSTHOG_KEY in environment to enable.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/store/authStore.ts

@@ -3,6 +3,7 @@ import { persist } from 'zustand/middleware'
 import * as Sentry from '@sentry/react'
 import type { User, Token, UserCreate, UserLogin, Account, SubscriptionDetails } from '@/types'
 import { authApi } from '@/api/auth'
+import { identifyUser, resetAnalytics, analytics } from '@/lib/analytics'
 import { apiClient } from '@/api/client'
 import { clearCachedQuota } from '@/hooks/useCachedQuota'
 
@@ -49,6 +50,7 @@ export const useAuthStore = create<AuthState>()(
 
           // Fetch user info
           await get().fetchUser()
+          analytics.loginSuccess()
         } catch (error: unknown) {
           const axiosErr = error as { response?: { data?: { detail?: unknown } } }
           const rawDetail = axiosErr.response?.data?.detail
@@ -62,6 +64,7 @@ export const useAuthStore = create<AuthState>()(
         set({ isLoading: true, error: null })
         try {
           await authApi.register(data)
+          analytics.accountCreated()
           // After registration, log the user in
           await get().login({ email: data.email, password: data.password })
         } catch (error: unknown) {
@@ -83,6 +86,7 @@ export const useAuthStore = create<AuthState>()(
           localStorage.removeItem('refresh_token')
           clearCachedQuota()
           Sentry.setUser(null)
+          resetAnalytics()
           set({ user: null, token: null, account: null, subscription: null, isAuthenticated: false, error: null })
         }
       },
@@ -109,6 +113,9 @@ export const useAuthStore = create<AuthState>()(
           // Set Sentry user context for error attribution
           Sentry.setUser({ id: user.id, email: user.email })
 
+          // Identify user in PostHog for product analytics
+          identifyUser({ id: user.id, email: user.email, role: user.role, is_super_admin: user.is_super_admin, account_id: account?.id })
+
           set({ user, account, subscription, isLoading: false })
         } catch (error: unknown) {
           const message = error instanceof Error ? error.message : 'Failed to fetch user'