fix(serve): serve stops its daemon (F2); BIOROUTER_SERVE_UI validated like --web-dir (F8); serve doc fixes (F10) - #226

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/