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]/StartDMButton.tsx

@@ -0,0 +1,36 @@
+"use client";
+
+import { useState } from "react";
+import { useRouter } from "next/navigation";
+import { MessageSquare } from "lucide-react";
+import { apiOpenDirectChat } from "@/lib/api";
+import { toast } from "@/lib/toast";
+
+/** 用户主页：发起私聊 */
+export default function StartDMButton({ userId }: { userId: number }) {
+  const router = useRouter();
+  const [loading, setLoading] = useState(false);
+
+  const start = async () => {
+    setLoading(true);
+    try {
+      const { room } = await apiOpenDirectChat(userId);
+      router.push(`/chat/${room.id}`);
+    } catch (e) {
+      toast(e instanceof Error ? e.message : "无法发起私聊");
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  return (
+    <button
+      onClick={start}
+      disabled={loading}
+      className="btn btn-accent btn-sm gap-1.5"
+    >
+      <MessageSquare size={14} />
+      {loading ? "打开中…" : "私聊"}
+    </button>
+  );
+}