I like it, the game is pretty well polished. How did you add 16000 endgame problems? Do these come from a database somewhere? I cannot imagine a single author adding 16000 endgame problems!
My only complaint is: the computer doesn't look that smart. I think some endgame problems are only possible because the computer makes a bad move, so the puzzle is about a chess problem involving a bad computer rather than perfect problems. E.g. the first problem of "beginner": if the computer were smarter, I think it could always prevent you from winning in 2 moves. Is that correct?