feat: add PostHog analytics integration

- Add PostHog client configuration in ui/lib/posthog.ts
- Integrate PostHog tracking in auth forms and provider workflows
- Add PostHog dependencies (posthog-js and posthog-node)
- Update Next.js config for PostHog integration
- Add environment variables for PostHog configuration
- Update changelog with PostHog integration entry

ui/package.json

@@ -39,6 +39,8 @@
     "next": "^14.2.30",
     "next-auth": "^5.0.0-beta.25",
     "next-themes": "^0.2.1",
+    "posthog-js": "^1.256.1",
+    "posthog-node": "^5.1.1",
     "radix-ui": "^1.1.3",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",