chore(assets): modernise frontend lint tooling (ESLint 10 flat config, Prettier 3, Stylelint 17) - #3775

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: cypress-io/github-action@v6. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/