Improve test coverage

[FrNecas]

Due to deadline pressure, test coverage was totally neglected in this project. However, moving forward, it would be useful to add some basic test coverage (and run it in a CI, e.g. Github Actions) to avoid unnecessary accidental breakages.

Adding tests to the backend should not be difficult, since it's structured quite nicely.

Frontend may be a different story though, the code is a bit of a mess in some places.