A prompt template with `{{flag | checkbox:default=false}}` opened the input variables form with the checkbox already ticked, and the same happened for `False`, `"false"`, `0` and `"0"`. Template defaults arrive as strings and the checkbox was bound straight to that value, so any non-empty string counted as checked and there was no way to define a checkbox that starts unchecked once a default was given.
The checkbox now reads its ticked state from the value explicitly: it is checked only for `true` (boolean or string) and stays unchecked for anything else, while clicking it still writes a boolean like before. The seeded value itself is left alone, so what gets substituted into the prompt for an untouched form is unchanged, and typing `true` or `false` into the text field next to the checkbox now moves the tick accordingly.
Fixes#30036
