refactor(permissions): remove hard-off compat stubs and harden login schema

Delete aksrt-compat.php and rewire messages/PM entry points to the user
center notification route. Normalize permissions login dependsOn objects
and allow an explicit empty captcha provider option so login/moderation
schema pages remain loadable without legacy compatibility shims.

inc/classes/admin/schema/class-aksrt-admin-schema-permissions.php

@@ -23,6 +23,7 @@ class AKSRT_Admin_Schema_Permissions {
                         ['key' => 'basic', 'title' => '基础开关', 'fields' => [
                             ['key' => 'enabled', 'label' => '启用人机验证', 'type' => 'switch', 'default' => false],
                             ['key' => 'active_provider', 'label' => '当前服务商', 'type' => 'radio', 'default' => '', 'dependsOn' => ['key' => 'enabled', 'value' => true], 'options' => [
+                                ['label' => '关闭', 'value' => ''],
                                 ['label' => '腾讯云 Captcha', 'value' => 'tencent'],
                                 ['label' => '阿里云 CAPTCHA 2.0', 'value' => 'aliyun'],
                                 ['label' => 'Cloudflare Turnstile', 'value' => 'cloudflare'],