fix(admin): create-playtest form validation + layout UX - #9

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

The 'applicants' logical expression could make the dependencies of useMemo Hook (at line 142) change on every render. Move it inside the useMemo callback. Alternatively, wrap the initialization of 'applicants' in its own useMemo() Hook