Several Cristal instances connected to different backends can co-exist

As a user, I can have several Cristal browser tabs or several Cristal desktop apps working at the same time and connected to different backends. The goal is to be able to parallelize work, in a way that's simpler than having to switch the backend configuration (which is also not possible in the case of CRISTAL-32 where you need this feature if you want to connect to several backends at the same time).