Hey, thanks for the response. About the leaderboards disappearing from the list, they are only saved as cache, so as soon as you clear the data of websites on your browser, they will be gone. That is why it is important to save the secret keys.
I think that I could create an account system in the future, which would just require logging in, and your leaderboards will be accessible at any time, from any device without restoring all of them.