functions.php

<?php
/**
 * Only Login theme — block public front-end except API-like requests.
 *
 * @package only_admin
 */

defined( 'ABSPATH' ) || exit;


add_action( 'template_redirect', 'only_admin_redirect_visitors', 0 );
/**
 * Send visitors to login or admin; do not trap REST or common API entry points.
 *
 * @return void
 */
function only_admin_redirect_visitors() {
	if ( defined( 'REST_REQUEST' ) && REST_REQUEST ) {
		return;
	}

	if ( function_exists( 'wp_is_serving_rest_request' ) && wp_is_serving_rest_request() ) {
		return;
	}

	global $wp;

	if ( ! empty( $wp->query_vars['rest_route'] ) ) {
		return;
	}

	// WooCommerce legacy REST entry point.
	if ( get_query_var( 'wc-api', false ) ) {
		return;
	}

	$request_uri = '';
	if ( isset( $_SERVER['REQUEST_URI'] ) ) {
		$request_uri = sanitize_text_field( wp_unslash( $_SERVER['REQUEST_URI'] ) );
	}

	if ( '' !== $request_uri && false !== strpos( $request_uri, 'wp-json' ) ) {
		return;
	}

	if ( is_user_logged_in() ) {
		wp_safe_redirect( admin_url() );
	} else {
		wp_safe_redirect( wp_login_url() );
	}
	exit;
}