feat: integrate PostHog analytics for frontend and PWA

- Install posthog-js and @posthog/react packages
- Add PostHogProvider wrapper in main.tsx for both apps
- Configure PostHog with EU instance and 2026-01-30 defaults
- Add environment variables for API key and host
- Update docker-compose.coolify.yml with PostHog build args

frontend/.env.example

@@ -11,3 +11,7 @@ VITE_API_URL=http://localhost:3000/api/v1
 
 # ===== COMMON =====
 VITE_CMS_URL=http://localhost:1337
+
+# ===== POSTHOG ANALYTICS =====
+VITE_PUBLIC_POSTHOG_KEY=phc_kuDGT1kUmJmijLRjiQaKWIaU4JBdFW0aELW2hIGlBXR
+VITE_PUBLIC_POSTHOG_HOST=https://eu.i.posthog.com