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.

templates/parts/single-author-bar.php

@@ -64,8 +64,8 @@ $classes = trim('article-avatar ' . $extra_class . ($show_actions ? ' article-av
 					<span><?php esc_html_e('关注', 'aksrt'); ?></span>
 				</button>
 				<?php // 私信为“停用候选”兼容能力：默认不在前台暴露，仅在站点显式开启时显示。 ?>
-				<?php if (function_exists('aksrt_pm_frontend_enabled') && aksrt_pm_frontend_enabled()) : ?>
-					<a href="<?php echo esc_url(add_query_arg('msg_type', 'conversation', aksrt_get_messages_url())); ?>" class="author-action-btn author-action-btn--ghost">
+				<?php if (is_user_logged_in()) : ?>
+					<a href="<?php echo esc_url(add_query_arg('msg_type', 'conversation', aksrt_get_user_center_url('notif'))); ?>" class="author-action-btn author-action-btn--ghost">
 						<i class="far fa-comment-dots" aria-hidden="true"></i>
 						<span><?php esc_html_e('私信', 'aksrt'); ?></span>
 					</a>