Hey! Good news — I've added this feature. Go to
Institutions → View to set it up:
https://bellocco.com/instellingen?tab=weergave
You can pick portrait or landscape and turn on the lock. Once enabled, a small lock icon will appear in the bottom-right corner while you're playing on mobile — tap it to lock your screen in place.
A couple of honest notes: it works best on Android/Chrome, since iPhone's Safari doesn't let websites control screen rotation at all (that's an Apple limitation, not something I can fix). Also, since every page load reloads the whole page, you'll need to tap the lock icon again after navigating to a new page — it's a one-tap thing though, not a big hassle. Let me know how it works for you!