docs: Add Mixpanel analytics configuration and documentation

- Add MIXPANEL_TOKEN, MIXPANEL_API_SECRET, ANALYTICS_ENV to .env.example
- Add packages/web/.env.example with VITE_MIXPANEL_TOKEN and other analytics vars
- Update docs/MIXPANEL_ANALYTICS.md with complete setup instructions
- Document event taxonomy (30+ events across User, Subscription, DarkWatch, VoicePrint, SpamShield)
- Add KPI definitions (MAU, MRR, conversion, churn, CAC, LTV, NPS, viral coefficient)
- Include integration examples for backend and frontend usage
- Document alert thresholds for monitoring

Implementation was already complete in packages/shared-analytics and packages/web.
This completes the configuration and documentation for Mixpanel setup.

FRE-5281

packages/web/.env.example

@@ -0,0 +1,31 @@
+# ShieldAI Web Application Environment Variables
+# Copy this file to .env and fill in the values
+
+# ============================================
+# Product Analytics
+# ============================================
+
+# Google Analytics 4
+VITE_GA_MEASUREMENT_ID=""
+
+# Mixpanel Product Analytics
+VITE_MIXPANEL_TOKEN=""
+
+# Meta (Facebook) Pixel
+VITE_META_PIXEL_ID=""
+
+# LinkedIn Insight Partner ID
+VITE_LINKEDIN_PARTNER_ID=""
+
+# ============================================
+# Application Configuration
+# ============================================
+
+# API Base URL
+VITE_API_BASE_URL=http://localhost:3000
+
+# Environment (development, production, staging)
+VITE_APP_ENV=development
+
+# App Version
+VITE_APP_VERSION=0.1.0