fix(流转树): 弹窗打开时锁定背景滚动，消除滚动穿透

- 通用 Modal 打开时 body overflow=hidden，关闭时恢复
- 修复流转树宽屏弹窗滚动时背景看板跟随滚动的问题
- TaskFlowView 记录弹窗同步加锁(覆盖无外层弹窗的页面内打开场景)

frontend/src/components/TaskTree/TaskFlowView.tsx

@@ -1,7 +1,7 @@
 /**
  * 流转树双模式可视化 — 焦点模式 + 全景模式
  */
-import { memo, useMemo, useState } from "react";
+import { memo, useMemo, useState, useEffect } from "react";
 import { FileText, X } from "lucide-react";
 import type { TaskResponse } from "../../types/api";
 import { TASK_STATUS } from "../../types/api";
@@ -154,6 +154,14 @@ export const TaskFlowView = memo(function TaskFlowView({ tasks, onAction, curren
   const [showFullMap, setShowFullMap] = useState(false);
   const [recordsTask, setRecordsTask] = useState<TaskResponse | null>(null);
 
+  // 🚀 记录弹窗打开时锁定背景滚动（防止滚动穿透），关闭时恢复
+  useEffect(() => {
+    if (!recordsTask) return;
+    const prev = document.body.style.overflow;
+    document.body.style.overflow = "hidden";
+    return () => { document.body.style.overflow = prev; };
+  }, [recordsTask]);
+
   // 数据分类 — 🚀 仅根级主干作为垂直时间线节点，子节点通过 childMap 分支递归渲染
   const { allMains, childMap } = useMemo(() => {
     ALL_TASKS.clear(); if (tasks.length) collectAll(tasks);