feat(ai-writing): 后台挂起改为即时返回，乐观标记历史「挂起中」

点击「转后台」后立刻 reset 回初始页，挂起请求 fire-and-forget；
历史下拉靠 markBackground/clearBackground 乐观标记马上显示「挂起中」，
不等后端确认，与「多智能体会商」交互一致。

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

frontend-web/src/open-canvas/hooks/useAIWritingSessions.ts

@@ -1,4 +1,4 @@
-import { useCallback, useState } from 'react';
+import { useCallback, useMemo, useState } from 'react';
 
 import {
   listAIWritingSessions,
@@ -10,6 +10,27 @@ import type { AIWritingSessionSummary } from '../api/ai-writing-sessions';
 export function useAIWritingSessions() {
   const [sessions, setSessions] = useState<AIWritingSessionSummary[]>([]);
   const [loading, setLoading] = useState(false);
+  // 「后台挂起」乐观标记：点击挂起后立刻让历史下拉显示「挂起中」，不必等后端确认。
+  // 后端状态（status='background'）刷新到位后由调用方 clearBackground 清除。
+  const [pendingBackground, setPendingBackground] = useState<Set<string>>(() => new Set());
+
+  const markBackground = useCallback((id: string) => {
+    setPendingBackground((prev) => {
+      if (prev.has(id)) return prev;
+      const next = new Set(prev);
+      next.add(id);
+      return next;
+    });
+  }, []);
+
+  const clearBackground = useCallback((id: string) => {
+    setPendingBackground((prev) => {
+      if (!prev.has(id)) return prev;
+      const next = new Set(prev);
+      next.delete(id);
+      return next;
+    });
+  }, []);
 
   const refresh = useCallback(async () => {
     setLoading(true);
@@ -41,5 +62,18 @@ export function useAIWritingSessions() {
     }
   }, []);
 
-  return { sessions, loading, refresh, rename, remove };
+  // 把乐观「挂起中」标记叠加到真实列表上：已在列表里的会话立刻显示 background。
+  const displaySessions = useMemo(
+    () =>
+      pendingBackground.size === 0
+        ? sessions
+        : sessions.map((s) =>
+            pendingBackground.has(s.id) && s.status !== 'done'
+              ? { ...s, status: 'background' }
+              : s,
+          ),
+    [sessions, pendingBackground],
+  );
+
+  return { sessions: displaySessions, loading, refresh, rename, remove, markBackground, clearBackground };
 }