Got the same feedback from a few other people, so I decided to go a bit further. I've now implemented local storage of rating and got rid of the signin requirement. Now you can play anonymously as long as you want and only need to sign in to preserve your rating if you switch browsers or devices or want to be on the leaderboard.

Thanks for the feedback. I've added a link to the tutorial on the signing screen so you can try out puzzles without signing in.

The rating system and ability to serve appropriate difficulty puzzles relies on the user being signed in, but I don't collect any personal info, just the user ID (no email, account name, or anything else).