I think I would be able to add that, also your project is saved every move, either when you move some clip on timeline or on canvas/player

Automatic saving seems like the best option. Then you can always just re-open the tab. Undo is better than confirmation.

FWIW a few people fly around the web with persistent cookies and localstorage disabled except for selected sites -- once the tab is closed, the data is flushed, and re-opening the tab won't restore it. I'm one of those people. I recognize that this is entirely a "me" problem and don't expect anyone to change the way their software works to accommodate this behavior. Just raising a flag.

I would use the following behavior: the first time (loaded with empty storage) ask for confirmation, the next times (if loaded from storage) don't ask.

This would prevent people with auto cleanup to loose their work and avoid annoying others too much.