lively.next looks exactly like something I’ve been thinking about. How come though that the demo video shows it in use on Linux but the desktop application is macOS only?
If you want to run it locally via https://github.com/LivelyKernel/lively.installer then it should run well in MacOS and Linux. On Windows you might need to use a proper bash (e.g. git bash or the Ubuntu subsystem).
There is a desktop app for mac os that wraps the installer and gives you a menu bar entry for controlling the lively server but in the end it is just a frontend for what the installer does.