Let users collapse the wide-mode NavigationRail to an icons-only compact
view to reclaim horizontal space. A bottom-pinned double-chevron toggle
sits centred when compact and slides to the rail's right side when
extended, animating in sync with the rail. The choice is persisted via
the nav_rail_extended preference (defaults to extended).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
