Gameplay input (finished?)

Your app must somehow indicate to the user which letters he or she has
(or, if you prefer, hasn’t) guessed yet.

The user must be able to input guesses via an on-screen keyboard.

Your app must only accept as valid input single alphabetical characters
(case-insensitively).

Invalid input (e.g., multiple characters, no characters, characters
already inputted, punctuation, etc.) should be ignored (silently or with
some sort of alert) but not penalized.

Your app must have a title (e.g., Hangman) or logo and one button that
starts a new game.