feat(client): route-aware breadcrumbs and multilevel sidebar from shared nav tree

Breadcrumbs: shadcn base breadcrumb rendered in SiteHeader, replacing the
hardcoded dashboard heading - every protected page gets a location trail
with zero per-page wiring.

Multilevel sidebar: NAV_ITEMS becomes a hierarchical registry consumed by
both the sidebar and the breadcrumb resolver, keeping labels and structure
consistent across the two surfaces. Groups render as collapsible sections
(base-ui Collapsible + SidebarMenuSub) that auto-open for the active
child; the group chevron points forward per direction via an rtl mirror
and rotates 90 degrees while expanded (shadcn convention).

Trail resolution: exact nav matches use each entry's lineage; other pages
anchor on the deepest covering nav section and append static page crumbs
(literal-first, :param fallback). Ancestors link; the last crumb marks the
current page.