BSidebar: Optional backdrop support

BSidebar: Optional backdrop support: a task in LegoFlow-SWE (Harbor dataset). Additionally, the document body must remain scrollable while the sidebar (with backdrop) is open, unlike the behaviour of modals that prevent body scrolling. The component should fully support the new features without…

The task

Additionally, the document **body** must remain scrollable while the sidebar (with backdrop) is open, unlike the behaviour of modals that prevent body scrolling. The component should fully support the new features without breaking existing usage where no `backdrop` prop is supplied.

Part of Lego-X/LegoFlow-SWE.