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/theme/page.tsx

@@ -15,10 +15,6 @@ import { listCustomThemes } from "@/lib/theme-custom-store";
 import { FONTS, PRESETS } from "@/lib/theme-presets";
 import { ColorField } from "./color-field";
 
-// 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: "Theme" };
 
 const COLOR_FIELDS: {