This is great!
I agree the input is (surprisingly, to me) super intuitive.
One suggestion is that I think you should make the "win" state a lot more obvious. You could maybe draw a big box with "you win" text over the center of the screen, (maybe erasing it after any input) ...or if you don't want to obscure the game, I could imagine drawing a black border around the whole board the same width as your top "header", (reversing the color of that text in the process).
My other suggestions (all sort of "wishlist" items of lesser importance) are all around meta-game stuff. For one thing, it would be nice to have a table of previous game times somewhere. (Which could potentially give you some objectives to shoot for if you pre-populate it with some values, maybe.) Another thing would be to have some help-text that explains the two game modes. Not sure where you would put either of these without making a real menu tho...
Anyway, excellent work!