Base application layout providing adaptive navigation: a bottom icon bar
with a secondary tab bar on phone, a permanent left-strip rail on tablet,
and a drawer-based SideNav on desktop.
Builds and updates the nav-item content for one of AppNavLayout's five
device/orientation scenarios (desktop, tablet portrait, tablet landscape, phone portrait,
phone landscape), placing it into whichever of AppNavLayout's named locations
(see NavSlots) is appropriate.