Is my Supernote password safe? Does the app store it?

Yes, it's safe. The app never handles or stores your password — you sign in on Supernote's own real login page using an emailed code. The app only keeps the logged-in session that Supernote hands back, the same way a browser stays logged in, so it can fetch your to-dos. Signing out clears that. You will never be asked to enter your password.