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/Modal.tsx

@@ -1,6 +1,7 @@
 "use client";
 
 import { useEffect, useRef, type PointerEvent, type ReactNode } from "react";
+import { createPortal } from "react-dom";
 import { X } from "lucide-react";
 
 export type ModalProps = {
@@ -102,7 +103,7 @@ export default function Modal({
     overlayPressedRef.current = false;
   };
 
-  return (
+  return createPortal(
     <div
       className={`fixed inset-0 ${zIndexClass} flex items-center justify-center p-4`}
       style={{ background: "rgba(0,0,0,0.55)" }}
@@ -150,7 +151,8 @@ export default function Modal({
           <div className="flex items-center justify-end gap-2 mt-5">{footer}</div>
         ) : null}
       </div>
-    </div>
+    </div>,
+    document.body
   );
 }