Prevent letters being entered in number fields

This code addresses a minor but important oversight in Knack. Currently, Knack allows characters to be entered into a field designated for numbers. While these characters are not saved in the database—only the numbers are—it can confuse front-end users because they receive no visual feedback indicating that letters are not permitted in these fields. The provided code rectifies this issue, enhancing the user experience by preventing the entry of non-numeric characters and thereby offering immediate, visual feedback to the user.