fix(chat): 发送后切会话不再误标未读，管理端撤回实时同步到对话页

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/components/PostLinkProvider.tsx

@@ -0,0 +1,20 @@
+"use client";
+
+import { createContext, useContext } from "react";
+
+const PostLinkCtx = createContext(true);
+
+/** 帖内 Markdown 外链是否新标签打开（来自公开站点设置） */
+export default function PostLinkProvider({
+  openInNewTab,
+  children,
+}: {
+  openInNewTab: boolean;
+  children: React.ReactNode;
+}) {
+  return <PostLinkCtx.Provider value={openInNewTab}>{children}</PostLinkCtx.Provider>;
+}
+
+export function usePostLinkNewTab() {
+  return useContext(PostLinkCtx);
+}