feat: AtchaV2 소셜 로그인 + 계정 수직 슬라이스 완결 (가입·로그아웃·탈퇴·유저정보) - #361

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/