feat(messages): 消息中心前台 1:1 导航/门控/资源加载

对齐 Zibll 消息中心主 tab 语义，补齐 messages/conversation 资源 enqueue，按 message_s/private_s 门控私信与总开关，并收束页面壳层与作者栏入口。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

templates/parts/single-author-bar.php

@@ -63,9 +63,12 @@ $classes = trim('article-avatar ' . $extra_class . ($show_actions ? ' article-av
 					<i class="far fa-heart" aria-hidden="true"></i>
 					<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 (is_user_logged_in()) : ?>
-					<a href="<?php echo esc_url(add_query_arg('msg_type', 'conversation', function_exists('aksrt_get_messages_page_url') ? aksrt_get_messages_page_url() : home_url('/'))); ?>" class="author-action-btn author-action-btn--ghost">
+				<?php
+				$aksrt_pm_on = class_exists('AKSRT_Settings') && !empty(AKSRT_Settings::private_message_settings()['enabled']);
+				$aksrt_msg_url = function_exists('aksrt_get_messages_page_url') ? aksrt_get_messages_page_url() : '';
+				if ($aksrt_pm_on && $aksrt_msg_url !== '') :
+				?>
+					<a href="<?php echo esc_url(add_query_arg('msg_type', 'conversation', $aksrt_msg_url)); ?>" 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>