Vlad Here to add my vote for this feature as I think it would be fantastic.
As some context, this is in addition from normal PWA's which can be installed from Chrome's url bar. Both Chrome and Edge have the option to create any webpage as a standalone app with its own icon in the dock, menubar name, etc. The benefit of this is the ability to have seperate apps for each site, but still use extensions installed in the normal browser. Here is a video showing an example of making this feedback site an app: Chrome Example - Streamable
iOS has this feature too, available using the Share Menu of Safari. It works best on pages that are PWA's as it creates them as a seperate app, rather than just acting as a bookmark to a Safari page. Here is an example video using Starbucks's Webapp: iOS Example - Streamable
Orion's implementation could copy from the iOS version by creating a new "Create App" option in the share menu. This would create a standalone app in
~/Orion Apps/. These apps would just be permanently in Focus Mode (possibly with a small titlebar just for window controls as suggested by @rikavoort).
Example of the Share Menu:
And the app would look like this when launched (just the site in focus mode):