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">;

examples/qwik/src/lib/bknd.ts

@@ -2,7 +2,7 @@ import { createBknd } from "bknd/adapter/universal";
 import bkndConfig from "../../bknd.config";
 import type { EnvGetter } from "@builder.io/qwik-city/middleware/request-handler";
 
-export const getApp = async (env?: EnvGetter) => createBknd({ mode: "standalone", options: bkndConfig }, env);
+export const getApp = async (env?: EnvGetter) => createBknd({ mode: "admin", options: bkndConfig }, env);
 
 export const handler = async (req: Request, env?: EnvGetter) => {
    return (await getApp(env)).serve()(req);