Editor: Add input for number for border size

Seems like along with the slider to select border size, an input field for a number would be a good feature

Start by locating the editor screen and the existing border-size slider. Read how the slider value is stored and applied, then determine where a numeric input belongs beside it. Done means users can enter a border size directly and the value is applied consistently with slider changes; add or update the relevant UI test if one exists.

Written by the indexing model from the issue text.