fix(roundtable): 会商第三步各席位产物弹窗经 portal 挂 body，修复顶部被顶栏盖住

页面 <main> 的 `relative z-0` 新建层叠上下文，把内部 fixed 弹窗困在 z-0 层级，
被会商页头(z-40)/全局顶栏(z-45)盖住标题栏。ContentModal 与 ArtifactPreviewModal
改用 createPortal 挂到 document.body，脱离该上下文后正常盖在顶栏之上。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/roundtable-planning/components/ArtifactPreviewModal.tsx

@@ -1,4 +1,5 @@
 import React, { useCallback, useEffect, useMemo, useState } from "react";
+import { createPortal } from "react-dom";
 import type { ReactNode } from "react";
 import ReactMarkdown from "react-markdown";
 import remarkGfm from "remark-gfm";
@@ -214,7 +215,9 @@ export const ArtifactPreviewModal: React.FC<ArtifactPreviewModalProps> = ({
     [content],
   );
 
-  return (
+  // 经 portal 挂到 body：脱离会商页 <main>(`relative z-0`) 的层叠上下文，
+  // 否则 fixed 弹窗会被会商页头(z-40)/全局顶栏(z-45)盖住顶部。
+  return createPortal(
     <div
       className="fixed inset-0 z-[100] flex items-center justify-center bg-slate-900/40 backdrop-blur-sm p-6"
       onMouseDown={(e) => {
@@ -287,6 +290,7 @@ export const ArtifactPreviewModal: React.FC<ArtifactPreviewModalProps> = ({
           )}
         </div>
       </div>
-    </div>
+    </div>,
+    document.body,
   );
 };