CI: improve UI-related CI jobs:`UI eslint` obscures TypeScript Compiler errors, `npm run build` runs too late

A TypeScript error now results in NO more linting, NOR Cypress testing:

And now the Cypress CI job is faster! It no longer runs npm run build, not even npm install — because it receives all that work from the earlier UI CI job 👍

Bonus: 📌 CI: do not run PHP-only jobs if an MR only changes the UI Active will improve the DX for JS UI-only changes further 😊

I see now the issue title was wrong — but the summary was right 🙈

Automatically closed - issue fixed for 2 weeks with no activity.