Function nav_scaffold 

pub fn nav_scaffold<R, E>(
    title: impl Into<String>,
    dark_mode: bool,
    tabs: Vec<Tab>,
    body: Widget,
    nav: &Nav<R>,
    on_back: E,
) -> Widget
where
    R: Clone + Serialize,
    E: Serialize,

Scaffold driven by a Nav stack: fills route (from the current route’s serialization) and depth (stack depth) so the shell animates transitions, and shows a back affordance (top-bar arrow + system back button) firing on_back whenever the stack can pop.