Feature Request: “flag” a kana

[KY246]

Is your feature request related to a problem? Please describe.
This isn't exactly a problem, but I thought that it would be easier to play the game if it was possible to flag kanas that you know can't be in the solution, similar to how flags are placed in minesweeper.

For example, if I try the two words,

あかるい
けんこう

and I get

⬛ ⬛ ⬛ ⇅
⬛ ⬛ ⇄ ⇅

I can deduce that the solution won't contain any of the characters in the かきくけこ column because the last column is a kana in the あいうえお column, and the vertical arrows didn't appear for か in 明るい, nor for the け and the こ in 健康. Therefore it would make sense if there was a way to mark kana which haven't actually been tried but you know are not a part of the answer.

Describe the solution you'd like
Some way to mark kana. I think that right-clicking to mark kana would be a good solution for computer users, but am unsure of how to implement something like this for mobile users.

Describe alternatives you've considered
A crude alternative that I use is a bookmarklet which allows me to change the colours of the kana that I can tell are wrong.

javascript:(function(){
  document.body.addEventListener("contextmenu", e => {
    e.preventDefault();
    if(e.target.tagName.toLowerCase() == "button"){
      if(e.target.style.background != "rgb(50, 21, 29)"){
        e.target.style.background = "rgb(50, 21, 29)";
      }else{
        e.target.style.background = "";
      }
    }
  });
})();