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/shell/AppShell.test.tsx

@@ -146,6 +146,15 @@ test("shell renders the overview route with all nav links", async () => {
 	}
 });
 
+test("main carries the ids the router scrolls and restores", async () => {
+	renderShell();
+	await screen.findByRole("heading", { name: "Overview" });
+
+	const main = screen.getByRole("main");
+	expect(main.id).toBe("main-content");
+	expect(main.getAttribute("data-scroll-restoration-id")).toBe("main");
+});
+
 test("the three configuration pages sit under a labelled group, after the rest", async () => {
 	renderShell();
 	await screen.findByRole("heading", { name: "Overview" });