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/TaskTreeViewer.tsx

@@ -1,4 +1,4 @@
-import { useState, useCallback, useMemo, memo } from "react";
+import { useState, useCallback, useMemo, useEffect, memo } from "react";
 import {
   Search,
   Loader2,
@@ -40,6 +40,14 @@ export const Modal = memo(function Modal({
   widthClass?: string;
   bodyClassName?: string;
 }) {
+  // 🚀 弹窗打开时锁定背景滚动（防止滚动穿透），关闭时恢复
+  useEffect(() => {
+    if (!open) return;
+    const prev = document.body.style.overflow;
+    document.body.style.overflow = "hidden";
+    return () => { document.body.style.overflow = prev; };
+  }, [open]);
+
   if (!open) return null;
 
   return (