Add a header toggle and a panel close button so the desktop file
sidebar collapses with a width transition, mirroring the journal
panel.
- split onClose (close button) from onNavigate (file selection) so
clicking a file no longer collapses the sidebar on desktop
- give the aside h-full so the file tree and outline keep bounded
scroll regions inside the new wrapper
- swap the aside shadow for a gray right border to match the journal