feat: 评论/帖子编辑历史与编辑标记，内容锁定

- 新增 smartDiff 差异计算与 EditHistoryModal 编辑历史弹窗，替换原 CommentEditHistoryModal
- 新增 PostEditedMark 编辑标记展示
- 新增内容锁定机制（content_lock）防止并发编辑冲突
- 审核与通知服务适配

frontend/components/NotificationBell.tsx

@@ -3,7 +3,7 @@
 import { useState, useEffect, useRef } from "react";
 import { useRouter } from "next/navigation";
 import Link from "next/link";
-import { Bell, CheckCheck, Heart, MessageCircle, CheckCircle2, XCircle, AtSign, ClipboardList, Award } from "lucide-react";
+import { Bell, CheckCheck, Heart, MessageCircle, CheckCircle2, XCircle, AtSign, ClipboardList, Award, AlertTriangle } from "lucide-react";
 import {
   apiFetchNotifications,
   apiUnreadCount,
@@ -137,17 +137,22 @@ export default function NotificationBell({ initialUnread = 0 }: { initialUnread?
     if (n.type === "pending_review") return "有新内容待审核";
     if (n.type === "deleted") return "删除了你的帖子";
     if (n.type === "badge") return "向你颁布了徽章";
+    if (n.type === "necro_reply") return "旧帖回复提醒";
     return "点赞了你的帖子";
   };
 
   // 审核结果类通知：无互动对象语义，使用专用图标与语义色
   const isApproval = (n: NotificationItem) =>
     n.type === "approved" || n.type === "rejected" || n.type === "deleted";
+  // 系统提醒（ActorID=0）：同样不渲染操作者链接
+  const isSystemNotice = (n: NotificationItem) => n.type === "necro_reply";
   const iconStyle = (n: NotificationItem): { color: string; bg: string; node: React.ReactNode } => {
     if (n.type === "approved")
       return { color: "var(--ok)", bg: "var(--ok-soft)", node: <CheckCircle2 size={15} /> };
     if (n.type === "rejected" || n.type === "deleted")
       return { color: "var(--danger)", bg: "var(--danger-soft)", node: <XCircle size={15} /> };
+    if (n.type === "necro_reply")
+      return { color: "var(--danger)", bg: "var(--danger-soft)", node: <AlertTriangle size={15} /> };
     if (n.type === "like")
       return { color: "var(--clay)", bg: "var(--clay-soft)", node: <Heart size={15} /> };
     if (n.type === "mention")
@@ -223,7 +228,7 @@ export default function NotificationBell({ initialUnread = 0 }: { initialUnread?
             ) : (
               notifications.map((n) => {
                 const ic = iconStyle(n);
-                const approval = isApproval(n);
+                const approval = isApproval(n) || isSystemNotice(n);
                 return (
                 <button
                   key={n.id}