Fix toolbar icon-only button vertical alignment

Overrides shiny's new .action-button .action-icon+.action-label margin-left
rule for toolbar icon-only buttons. This prevents the hidden label element
from affecting the button's vertical alignment.

The shiny PR #4350 added margin-left to .action-label elements to fix
underline rendering issues. However, for bslib's icon-only toolbar buttons,
the label is hidden but still present in the DOM, causing the margin to
create unexpected vertical alignment issues when combined with the minified
HTML output.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: elnelson575