admin: only the content region scrolls on wide screens

the shell grid grew past the viewport and scrolled the document,
carrying the sidebar with it. the shell is now viewport-height at the
wide breakpoint with main as the sole scroll container; the nav list
scrolls inside the pinned rail; router scroll restoration targets the
inner scroller so navigation resets it and back/forward restores it.

admin/src/routes.tsx

@@ -425,6 +425,8 @@ export function createAppRouter(history?: RouterHistory, queryClient: QueryClien
 		context: { queryClient },
 		defaultPreload: "intent",
 		defaultPreloadStaleTime: 0,
+		scrollRestoration: true,
+		scrollToTopSelectors: ["#main-content"],
 		defaultPendingComponent: RoutePending,
 		defaultErrorComponent: RouteError,
 	});