feat: 스택 잔여분 env/dev 일괄 반영 — Phase 14~18 + 레거시 스플래시 비주얼 (머지 순서 수습) - #358

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/