Convenience accessor for the sidenav configuration cascade. Runs in injection
context; resolves through the priority chain (per-instance Input ->
provideSidenavConfigAt -> provideSidenavConfig -> library defaults).
Equivalent to inject(CNGX_SIDENAV_CONFIG) - the helper exists so consumers
don't import the token directly. Mirrors injectBreadcrumbConfig in
@cngx/ui/breadcrumb.