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/lib/workbench/useChapterListCollapse.test.ts

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+// @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

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+"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 };
+}