feat: 实时消息与管理端运营能力

新增群聊/私信与公共 WebSocket 总线，站长用户内容档案，并将通知与待审角标改为推送驱动；同步精简管理仪表盘并修复主题换肤 DOM 冲突。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/app/u/[id]/page.tsx

@@ -14,6 +14,7 @@ import PasswordForm from "./PasswordForm";
 import Avatar from "@/components/Avatar";
 import RoleBadge from "@/components/RoleBadge";
 import { roleMeta } from "@/lib/roles";
+import StartDMButton from "./StartDMButton";
 
 interface UserPageProps {
   params: Promise<{ id: string }>;
@@ -132,8 +133,8 @@ export default async function UserPage({ params, searchParams }: UserPageProps)
           />
         </div>
 
-        {/* 本人操作按钮 */}
-        {isOwner && (
+        {/* 本人操作按钮 / 访客私聊 */}
+        {isOwner ? (
           <div className="absolute top-3 right-3 sm:top-4 sm:right-4 z-20 flex items-center gap-2">
             <Link href={`/u/${id}?tab=settings`} className="btn btn-line btn-sm">
               <PenLine size={13.5} />
@@ -148,7 +149,11 @@ export default async function UserPage({ params, searchParams }: UserPageProps)
               <Shield size={14} />
             </Link>
           </div>
-        )}
+        ) : currentUser ? (
+          <div className="absolute top-3 right-3 sm:top-4 sm:right-4 z-20">
+            <StartDMButton userId={data.user.id} />
+          </div>
+        ) : null}
 
         {/* 身份信息：纯文档流，头像 + 右侧文字 baseline 对齐 */}
         <div className="relative z-10 px-5 sm:px-7 pt-5 sm:pt-6">