refactor(autoload): migrate theme classes to Composer PSR-4

Move 288 types from global AKSRT_* + inc/classes manual requires into
namespace AKSRT\{Domain} under src/AKSRT/, wire composer.json psr-4,
and slim bootstrap to helpers/modules/hooks only. Split multi-class
widget/gateway files, update admin contract audit paths, and document
the layout in ADR/README.

404.php

@@ -9,8 +9,8 @@ defined('ABSPATH') || exit;
 
 get_header();
 
-$other_page_settings = class_exists('AKSRT_Settings')
-	? AKSRT_Settings::get_other_page_settings()
+$other_page_settings = class_exists('\AKSRT\Settings\Settings')
+	? \AKSRT\Settings\Settings::get_other_page_settings()
 	: [
 		'page_404_message' => __('页面未找到', 'aksrt'),
 		'page_404_search'  => true,