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.

inc/helpers/user-center-renderers/settings-bindings.php

@@ -9,7 +9,7 @@
 
 defined('ABSPATH') || exit;
 
-$oauth = class_exists('AKSRT_OAuth') ? AKSRT_OAuth::get_instance() : null;
+$oauth = class_exists('\AKSRT\Auth\OAuth') ? \AKSRT\Auth\OAuth::get_instance() : null;
 $oauth_data = $oauth ? $oauth->get_frontend_bindings_data($user_id) : ['bindings' => [], 'available' => []];
 $binding_name_map = [
 	'github' => 'GitHub',