Improve the Review forgotten cards dialog

The Review forgotten cards dialog

• Add a title "Review forgotten cards".
• Remove "x days" from the description.
• Add a unit "day"/"days" to the box.
  • I decided against input steppers since this input isn't a saved setting. It also allows to add the unit more organically.
• Replace "OK" with "Create".
• If the resulting filtered deck is empty (i.e., no cards match the criteria), a red warning should appear and the button should be disabled, instead of an error dialog (see reference).
• Show a warning when the input is invalid, limit the digits, and prevent leading zeros, like in https://github.com/ankidroid/Anki-Android/pull/20520, except without input steppers.

Reference dialogs