BUG: Starting dB input validation

The input for starting dB is meant to validate input to be an `int` between 0-120. It currently stops users from clearing the box and typing a number as it tries to keep at least a 0 present at all times.

Start in the Qt5 application's starting dB input handling and trace how the field enforces an integer from 0 to 120. Verify that the field can be cleared and that entering a valid number works without forcing 0 to remain visible; no specific file or test is named in the issue.

Written by the indexing model from the issue text.