Refactor application structure: remove unused assets and styles, implement routing with React Router, and add Footer component - #5

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-python@v5, peter-evans/create-or-update-comment@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/