[FEAT] 입력 검증 디바운스 + 붉은 테두리·카운터 현출 - #480

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, actions/github-script@v7, actions/setup-node@v4, aws-actions/configure-aws-credentials@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/