First: I am really spoiled by the great https://sudokupad.app/ which I used to play Fog-of-War sudokus from cracking the cryptic - see YouTube. So, you did really well to make a full-working game, but spoiled as I am, I am missing some functions. You do not have to implement them, I am just offering constructive, potential ideas here - not destructive criticism. When solving a sudoku, along the way, you need ways to enter your thoughts or hints, not only solving the cell with the solution digit. I always use Snyder notation.
Sudokupad offers different ways to have smaller digits in the cell: on the edges and in the center. Please try it out with some classic sudoku and look up Snyder notation if you don't know it yet. Also: Feel free to chat with me on discord if you like. Remember, just ideas no front.
Also: there is a special button/key to check if the entered big digits are correct. It's not needed, but I like it that way.
Entering an 8 with the number 8 is nice. You should also allow pressing 8 to remove it again (and maybe also BACKSPACE in addition to DEL).