Thank you very much!
Yes, you guessed it right, power button is quit. At the beginning, we didn't thought about browser version and needed quit button. But when I read your comment, actually we needed to add confirm popup.
Also we wanted to make leaderboard but we tried to use Godot 4 beta, it has cool features but some features was missing. So unfortunately we didn't want to continue developing with Godot 4.
Thank you for your feedbacks!