[Suggestion] Add a “black border” achievement/reward

[FuecocoBoi09]

(Terminology taken from BTD6 Community) Add a reward or achievement for completing Classic Mode without having to retry last battle or save scum. Proof of change in-game: https://www.youtube.com/watch?v=i-qB0PF8clE (Video by ISAB)

[j-diefenbach]

Might be a bit hard to implement / check properly since you could always just refresh and it will restart the battle