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>

assets/css/frontend/messages.css

@@ -578,3 +578,42 @@ html.dark .aksrt-pm__item.is-unread,
 html.dark .aksrt-pm__item.is-unread {
     background: rgba(240, 68, 148, 0.08);
 }
+
+
+/* Page header */
+.messages-center__header {
+    margin: 8px 0 16px;
+}
+.messages-center__title {
+    margin: 0 0 6px;
+    font-size: 22px;
+    font-weight: 700;
+    color: var(--color-ink, #1f2937);
+}
+.messages-center__subtitle {
+    margin: 0;
+    font-size: 14px;
+    color: var(--color-ink-3, #6b7280);
+}
+
+.messages-center--disabled {
+    display: block;
+    min-height: 40vh;
+}
+
+/* PM thread uses hidden attribute instead of inline style */
+.aksrt-pm__thread[hidden] {
+    display: none !important;
+}
+
+.message-item__new-badge {
+    display: inline-block;
+    margin-left: 6px;
+    padding: 1px 6px;
+    border-radius: 999px;
+    background: var(--color-primary, #3b82f6);
+    color: #fff;
+    font-size: 11px;
+    font-weight: 600;
+    vertical-align: middle;
+}