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);
+  });
+});