fix: move Monaco context menu to body-level DOM node
Use overflowWidgetsDomNode option to append Monaco's overflow
widgets (including context menu) to a container on <body>, fully
outside the editor panel hierarchy. This escapes all ancestor
stacking contexts so the menu renders on top of everything.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>