Refactor: Rename api mode to headless

The 'api' mode is being renamed to 'headless' to better reflect its
purpose: serving the API without the admin UI. This change aligns with
the existing framework concept of a headless backend.

The 'standalone' mode is renamed to 'admin' to indicate that it serves
both the API and the admin UI.

examples/qwik/bknd.config.ts

@@ -57,4 +57,4 @@ export default {
       assetsPath: "/admin/",
       logoReturnPath: "../..",
    },
-} satisfies Config<"standalone">;
+} satisfies Config<"admin">;