fix(captcha): close human-verification loop end-to-end

Rewrite captcha core around aksrt_captcha_config only, require captcha
on login/comment/SMS/phone auth and related gates, wire frontend token
collection, and remove dead dual-source security settings plus slider enqueue.

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

@@ -137,7 +137,7 @@ class AKSRT_Admin_Schema_Permissions {
                             ['key' => 'download_role_size', 'label' => '角色文件大小限制', 'type' => 'textarea', 'default' => '', 'help' => '格式: role|max_size_mb，每行一条'],
                         ]],
                         ['key' => 'captcha', 'title' => '安全设置', 'fields' => [
-                            ['key' => 'download_captcha', 'label' => '下载需要验证码', 'type' => 'switch', 'default' => false],
+                            ['key' => 'download_captcha_hint', 'label' => '下载验证码', 'type' => 'notice', 'alertType' => 'info', 'alertContent' => '下载验证码门槛在「功能&权限 → 下载权限」页（cms_features.download_captcha）维护；服务商与场景在「登录权限」。'],
                             ['key' => 'download_ip_limit', 'label' => 'IP下载限制', 'type' => 'number', 'default' => 0, 'min' => 0, 'help' => '同一IP每小时最大下载次数，0为不限制'],
                         ]],
                     ],