fix: context menu renders above gutters + username button keeps size
- Add overflowingContentWidgets selector and position:relative to
Monaco overlay z-index rule so context menu escapes stacking contexts
- Remove padding/border-width shrink on user menu in narrow/mobile
layouts so the username button stays full size
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>