It isn't impressive to spend a lot of time on a hiring problem, you shouldn't do that. If you can't do it in a few hours then just move on and apply for another job, you aren't the person they are looking for.

Doing it slowly over many days is only taking your time and probably wont get you the job anyway since the solution will be a hard to read mess compared to someone who solves it quickly since they are familiar with the domain.

The other comments here note that, and the author even stated it directly, that it was vibe-coded.

No. Should I invoice them? I'm still livid about it. The kicker is the position pays a max of 60-120k euros, the maximum being what I made 5 years ago.