Border-collapse stacked guess-entry buttons

When guess-entry buttons are stacked (i.e. on small viewports),
collapse the borders between them.

Inspired by SUIT CSS's `.ButtonGroup-borderCollapse`:
https://github.com/suitcss/components-button-group/blob/ed4a8bf6b4a157344953a8f6aba6fdb64563aec6/button-group.plugin.css#L18