The vertical navigation list: menu items from plain maps, current-page highlighting, groups, and nested sub-menus.
This is the list, not the chrome around it. For the app-shell furniture a nav
list usually lives in - the rail, the collapse to icons, the mobile sheet -
reach for PetalComponents.Sidebar.sidebar_nav/1; menu is the list it holds. The
two compose: a vertical_menu/1 drops straight into a sidebar's content area.
And a menu is for pages you NAVIGATE to. Hierarchical data you explore -
files, folders, an org chart - belongs in PetalComponents.Tree.tree/1,
a different widget with a different keyboard grammar. The sidebar holds
either; they never hold each other.