feat(frontend): 「目录」列可折叠、记忆折叠状态（UX P2-2）

useChapterListCollapse 用 localStorage 记忆折叠态（SSR 首帧恒展开避免 hydration 不一致）；收起降为带展开按钮的窄轨、让宽给正文，展开时标题旁加收起按钮；三栏栅格据折叠态切列宽，章节列表/高亮/切章导航全保留。

apps/web/components/workbench/ChapterList.tsx

@@ -1,6 +1,8 @@
 import Link from "next/link";
+import { PanelLeftClose, PanelLeftOpen } from "lucide-react";
 
 import { buildChapterEntries, type ChapterEntry } from "@/lib/workbench/chapter";
+import { focusRing } from "@/lib/ui/variants";
 
 interface ChapterListProps {
   projectId: string;
@@ -9,36 +11,83 @@ interface ChapterListProps {
   currentChapterNo: number;
 }
 
-// 左栏目录（UX §6.3）：列出大纲全部章节 + 当前章，点击切换写作章号（`?chapter=N`）。
-// 桌面 aside 包裹；移动端经 Workbench 抽屉复用 ChapterListContent。
+interface DesktopChapterListProps extends ChapterListProps {
+  // 目录列收起态（P2-2）：收起时只留一条带「展开」按钮的窄轨，让宽给正文。
+  collapsed: boolean;
+  onToggle: () => void;
+}
+
+// 左栏目录（UX §6.3 / P2-2）：列出大纲全部章节 + 当前章，点击切换写作章号（`?chapter=N`）。
+// 桌面 aside 包裹，可收起为窄轨；移动端经 Workbench 抽屉复用 ChapterListContent（不带折叠）。
 export function ChapterList({
   projectId,
   chapters,
   currentChapterNo,
-}: ChapterListProps) {
+  collapsed,
+  onToggle,
+}: DesktopChapterListProps) {
+  if (collapsed) {
+    return (
+      <aside
+        className="hidden shrink-0 border-r border-line bg-panel py-4 xl:block"
+        aria-label="目录（已收起）"
+      >
+        <button
+          type="button"
+          onClick={onToggle}
+          aria-expanded={false}
+          aria-label="展开目录"
+          title="展开目录"
+          className={`mx-auto flex h-8 w-8 items-center justify-center rounded text-ink-soft transition-colors hover:bg-[var(--color-cinnabar-wash)] hover:text-cinnabar ${focusRing}`}
+        >
+          <PanelLeftOpen className="h-4 w-4" aria-hidden="true" />
+        </button>
+      </aside>
+    );
+  }
   return (
     <aside className="hidden border-r border-line bg-panel py-4 xl:block">
       <ChapterListContent
         projectId={projectId}
         chapters={chapters}
         currentChapterNo={currentChapterNo}
+        onCollapse={onToggle}
       />
     </aside>
   );
 }
 
+interface ChapterListContentProps extends ChapterListProps {
+  // 桌面列传入 → 目录标题旁显示「收起」按钮；移动抽屉不传（抽屉自带关闭）。
+  onCollapse?: () => void;
+}
+
 export function ChapterListContent({
   projectId,
   chapters,
   currentChapterNo,
-}: ChapterListProps) {
+  onCollapse,
+}: ChapterListContentProps) {
   const entries = buildChapterEntries(chapters, currentChapterNo);
   const hasOutline = chapters.length > 0;
   return (
     <>
-      <h2 className="px-4 text-xs font-semibold uppercase tracking-wide text-ink-soft">
+      <div className="flex items-center justify-between px-4">
+        <h2 className="text-xs font-semibold uppercase tracking-wide text-ink-soft">
           目录
         </h2>
+        {onCollapse ? (
+          <button
+            type="button"
+            onClick={onCollapse}
+            aria-label="收起目录"
+            title="收起目录"
+            className={`flex h-6 w-6 items-center justify-center rounded text-ink-soft transition-colors hover:bg-[var(--color-cinnabar-wash)] hover:text-cinnabar ${focusRing}`}
+          >
+            <PanelLeftClose className="h-4 w-4" aria-hidden="true" />
+          </button>
+        ) : null}
+      </div>
       <ul className="mt-2">
         {entries.map((entry) => {
           const active = entry.no === currentChapterNo;

apps/web/components/workbench/Workbench.tsx

@@ -59,6 +59,7 @@ import { GenrePicker } from "./GenrePicker";
 import { GENRES } from "@/lib/wizard/wizard";
 import { useGenreGate, toGenreDirective } from "@/lib/workbench/useGenreGate";
 import { useAiConversation } from "@/lib/workbench/useAiConversation";
+import { useChapterListCollapse } from "@/lib/workbench/useChapterListCollapse";
 
 interface WorkbenchProps {
   project: ProjectResponse;
@@ -104,6 +105,8 @@ export function Workbench({
   // AC-3 AI 对话抽屉：Workbench 级单实例（append 即使抽屉关着也能触发）；flag 灰度。
   const [conversationOpen, setConversationOpen] = useState(false);
   const conversation = useAiConversation(project.id, chapterNo);
+  // P2-2：桌面「目录」列可收起（记忆状态）；收起时把宽让给正文，窄轨留一个展开入口。
+  const chapterList = useChapterListCollapse();
   const conversationTriggerRef = useRef<HTMLButtonElement>(null);
   const toast = useToast();
   const autosave = useAutosave(project.id, chapterNo, initialText);
@@ -292,11 +295,19 @@ export function Workbench({
       projectId={project.id}
       activeNav="write"
     >
-      <div className="grid min-h-[calc(100vh-var(--chrome,4rem))] grid-cols-1 xl:h-[calc(100vh-var(--chrome,4rem))] xl:grid-cols-[12rem_1fr_18rem]">
+      <div
+        className={`grid min-h-[calc(100vh-var(--chrome,4rem))] grid-cols-1 xl:h-[calc(100vh-var(--chrome,4rem))] ${
+          chapterList.collapsed
+            ? "xl:grid-cols-[2.75rem_1fr_18rem]"
+            : "xl:grid-cols-[12rem_1fr_18rem]"
+        }`}
+      >
         <ChapterList
           projectId={project.id}
           chapters={chapters}
           currentChapterNo={chapterNo}
+          collapsed={chapterList.collapsed}
+          onToggle={chapterList.toggle}
         />
 
         <section className="flex min-w-0 flex-col bg-bg">

apps/web/lib/workbench/useChapterListCollapse.test.ts

@@ -0,0 +1,64 @@
+// @vitest-environment jsdom
+import { act, renderHook } from "@testing-library/react";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+
+import {
+  CHAPTER_LIST_COLLAPSE_KEY,
+  useChapterListCollapse,
+} from "./useChapterListCollapse";
+
+// 本仓库 jsdom 未提供可用的 Storage（setItem/clear 缺失），装一个内存版 localStorage。
+function installMemoryStorage(): Storage {
+  const map = new Map<string, string>();
+  const storage: Storage = {
+    get length() {
+      return map.size;
+    },
+    clear: () => map.clear(),
+    getItem: (key) => (map.has(key) ? (map.get(key) ?? null) : null),
+    key: (index) => Array.from(map.keys())[index] ?? null,
+    removeItem: (key) => map.delete(key),
+    setItem: (key, value) => map.set(key, String(value)),
+  };
+  Object.defineProperty(window, "localStorage", {
+    value: storage,
+    configurable: true,
+  });
+  return storage;
+}
+
+describe("useChapterListCollapse", () => {
+  beforeEach(() => {
+    installMemoryStorage();
+  });
+  afterEach(() => {
+    window.localStorage.clear();
+  });
+
+  it("默认展开（无存档时 collapsed=false）", () => {
+    const { result } = renderHook(() => useChapterListCollapse());
+    expect(result.current.collapsed).toBe(false);
+  });
+
+  it("toggle 翻转并把状态写入 localStorage", () => {
+    const { result } = renderHook(() => useChapterListCollapse());
+
+    act(() => result.current.toggle());
+
+    expect(result.current.collapsed).toBe(true);
+    expect(window.localStorage.getItem(CHAPTER_LIST_COLLAPSE_KEY)).toBe("1");
+
+    act(() => result.current.toggle());
+
+    expect(result.current.collapsed).toBe(false);
+    expect(window.localStorage.getItem(CHAPTER_LIST_COLLAPSE_KEY)).toBe("0");
+  });
+
+  it("挂载时从 localStorage 恢复已收起状态", () => {
+    window.localStorage.setItem(CHAPTER_LIST_COLLAPSE_KEY, "1");
+
+    const { result } = renderHook(() => useChapterListCollapse());
+
+    expect(result.current.collapsed).toBe(true);
+  });
+});

apps/web/lib/workbench/useChapterListCollapse.ts

@@ -0,0 +1,46 @@
+"use client";
+
+import { useCallback, useEffect, useState } from "react";
+
+// 「目录」列折叠偏好的持久化键（对齐既有 `ww.` 前缀约定）。
+export const CHAPTER_LIST_COLLAPSE_KEY = "ww.workbench_chapter_list_collapsed";
+
+export interface ChapterListCollapse {
+  // true=目录列收起（让宽给正文）；false=展开。
+  collapsed: boolean;
+  toggle: () => void;
+}
+
+function persist(collapsed: boolean): void {
+  if (typeof window === "undefined") return;
+  try {
+    window.localStorage.setItem(CHAPTER_LIST_COLLAPSE_KEY, collapsed ? "1" : "0");
+  } catch {
+    // 隐私模式/配额异常忽略：折叠是纯 UI 偏好，写盘失败不影响导航功能。
+  }
+}
+
+// 记住「目录」列收起/展开状态（P2-2）：SSR 首帧恒展开（避免 hydration 不一致），
+// 挂载后再从 localStorage 恢复；toggle 即时写回。仅桌面（≥xl）用得上，窄屏走抽屉不受影响。
+export function useChapterListCollapse(): ChapterListCollapse {
+  const [collapsed, setCollapsed] = useState(false);
+
+  useEffect(() => {
+    if (typeof window === "undefined") return;
+    try {
+      setCollapsed(window.localStorage.getItem(CHAPTER_LIST_COLLAPSE_KEY) === "1");
+    } catch {
+      // 读盘失败则维持默认展开。
+    }
+  }, []);
+
+  const toggle = useCallback(() => {
+    setCollapsed((prev) => {
+      const next = !prev;
+      persist(next);
+      return next;
+    });
+  }, []);
+
+  return { collapsed, toggle };
+}