📚 Update documentation with Headplane UI access and configuration

- Update INSTALLATION_GUIDE.md:
  * Add Headplane configuration section with proper config.yaml
  * Update docker-compose.yml example with simplified environment variables
  * Add Headplane login instructions and API key information
  * Update troubleshooting section for cookie_secret validation errors
- Update CLIENT_CONNECTION_TEST.md:
  * Add Headplane web UI access information
  * Include external access URL (192.168.0.151:3000/admin/)
  * Add login credentials and API key details
- Update start.sh:
  * Include Headplane UI URLs in installation summary
  * Add external access information for network environments

All documentation now reflects the working Headplane UI configuration
and provides complete setup instructions for both local and external access.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

CLIENT_CONNECTION_TEST.md

@@ -291,5 +291,11 @@ Headscale 서버와 Tailscale 클라이언트 간의 연결이 완벽하게 성
 - ✅ IP 주소 자동 할당 성공
 - ✅ 실시간 통신 확인
 - ✅ Headscale 관리 인터페이스 정상 동작
+- ✅ Headplane 웹 UI 외부 접속 성공
+
+### 🌐 완전한 관리 환경 구축
+- **Headscale API**: http://localhost:8070 (명령줄 관리)
+- **Headplane UI**: http://192.168.0.151:3000/admin/ (웹 관리)
+- **로그인 API Key**: `8qRr1IB.tV95CmA0fLaCiGGIgBfeoN9daHceFkzI`
 
 이제 **Tailscale을 완전히 대체**할 수 있는 자체 호스팅 VPN 솔루션이 구축되었습니다!