📚 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>

start.sh

@@ -68,7 +68,8 @@ echo "🎉 설치 완료!"
 echo ""
 echo "📋 접속 정보:"
 echo "  - Headscale API: http://localhost:8070"
-echo "  - Headplane UI:  http://localhost:3000"
+echo "  - Headplane UI:  http://localhost:3000/admin/"
+echo "  - 외부 접속:     http://192.168.0.151:3000/admin/"
 echo "  - API 키: $API_KEY"
 echo ""
 echo "📖 다음 단계:"