Not sure even how to ask a question as the answer may depend on WM or DE so I'm not sure if there is a common answer but my plan is to create an application that will limit screen time for my kids. They are still using Windows but soon I'm planning to migrate to Linux and there is no such solution with web access/gui/API and the central server. I made such a solution on Windows but I don't know even how to start with Linux.
I need to create a window that will cover content on all displays, which User will not be able to close/move/hide. Most probably I will use Cinnamon or KDE as DE. System service should be able to create a such window / run application in User Session. As a root, I will have all the needed access.