Add extra checks to CI

_Originally: https://github.com/virge-io/pricelist-frontend/issues/454_

For both BE & FE, though below I only wrote it down for FE tools:

(for FE, start from nextjs branch to avoid conflicts!)

Add the following to CI / package.json (if missing):

- [x] linting
- [x] add script to package.json to run eslint
- [x] settings should not conflict with prettier
- [x] add step to CI
- [x] unit tests:
- [x] setup vitest (and research alternatives for stageverslag!)
- [x] add first initial unit tests for a helper function and for a jsx/tsx component
- [x] add step to CI that runs unit tests
- [x] add coverage reporting
- [x] extra command in package.json
- [x] extra step in CI
- [x] configurable threshold via config, e.g. X% => fail CI if lower than X%
- [x] npm audit to check for security vulnerabilities
- [x] extra step in CI (at the end), or even separate file