perf: migrate pages to Cache Components via root layout opt-out

Remove the per-route 'export const instant = false' opt-outs now that the root layout carries the single Cache Components opt-out. Child pages inherit the opt-out, so admin/mod/radio leaf pages that only access cached or DB data stay instant while runtime-dependent pages remain dynamic. Update the staff-smoke contract test to assert the root-layout contract.

src/app/admin/menu/page.tsx

@@ -11,10 +11,6 @@ import { canAccess, getAdminContext, PERMS } from "@/lib/permissions";
 import { siteSettings } from "@/lib/services/site-settings";
 import { AdminMenuEditor } from "./admin-menu-editor";
 
-// TODO: Cache Components adoption. Refactor this route so this opt-out can be removed.
-// See: https://nextjs.org/docs/app/guides/migrating-to-cache-components
-export const instant = false;
-
 export const metadata = { title: "Admin menu" };
 
 export default async function AdminMenuPage() {