Nav destinations of the org admin shell. Label + icon mirror the personal
shell's navDestinations; 5 entries for Dashboard, Members, Assignments,
Analytics, and Org Settings. Shared with OrgDetailScaffold so detail
pages render an identical bottom-nav / rail.
Returns the selected index for the org bottom-nav / rail, or null when
the current route is a detail screen (/org/:orgId/notifications,
/org/:orgId/teams, etc.) that doesn't correspond to any of the five
shell destinations. Callers that need a non-null value (e.g. the main
shell) should fall back to 0 (Dashboard).