fix: revert to normal Monaco context menu with high z-index on editor panel
Remove overflowWidgetsDomNode hack (shadow DOM blocked external styling).
Instead give .editor-panel z-index:100 so the context menu renders inside
Monaco with full theme styling and appears above sibling gutters/panels.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>