fix: Custom usernames are possible

Description
It's possible to use a custom username by utilizing the browser dev tools.

Steps To Reproduce

1. Open Dev Tools
2. Go to Application
3. Go to Local Storage
4. Edit value of flutter.__nickname_collection_key__ such that the new custom name replaces the old one

Expected Behavior
Detection of tampering, maybe assigning a new random name?

Screenshots

[githubmonkey]

Good catch, thanks for reporting! I have plans to move all data to firebase after the competition anyway, which will put a stop to this hack.