feat(ui): 工作台精修 + 拆分（P3-3..7）

- Workbench.tsx 937→467 行，抽出 AiToolbar/WorkbenchToolbar/MobileContextBar
- P3-3 AI 工具条分组留白 + 眉题 + 主 CTA 凸显
- P3-4 专注写作模式（useFocusMode hook+vitest，隐藏侧栏加宽正文）+ 左双栏面色区分
- P3-5 右栏本章参考小标题去衬线(font-sans) + 空态引导(无标题保 h2→h3 大纲)
- P3-6 底栏信息/操作/状态三区 + TOC 当前章高亮；TOC 状态点因无每章状态数据跳过
- P3-7 展示层流式跟随滚动（不触碰 SSE 逻辑，尊重 reduced-motion）
- AppShell 加可选 hideNav（加法式，默认 false）

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

@@ -0,0 +1,61 @@
+// @vitest-environment jsdom
+import { act, renderHook } from "@testing-library/react";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+
+import { FOCUS_MODE_KEY, useFocusMode } from "./useFocusMode";
+
+// 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("useFocusMode", () => {
+  beforeEach(() => {
+    installMemoryStorage();
+  });
+  afterEach(() => {
+    window.localStorage.clear();
+  });
+
+  it("默认关闭（无存档时 focus=false）", () => {
+    const { result } = renderHook(() => useFocusMode());
+    expect(result.current.focus).toBe(false);
+  });
+
+  it("toggle 翻转并把状态写入 localStorage", () => {
+    const { result } = renderHook(() => useFocusMode());
+
+    act(() => result.current.toggle());
+
+    expect(result.current.focus).toBe(true);
+    expect(window.localStorage.getItem(FOCUS_MODE_KEY)).toBe("1");
+
+    act(() => result.current.toggle());
+
+    expect(result.current.focus).toBe(false);
+    expect(window.localStorage.getItem(FOCUS_MODE_KEY)).toBe("0");
+  });
+
+  it("挂载时从 localStorage 恢复已开启状态", () => {
+    window.localStorage.setItem(FOCUS_MODE_KEY, "1");
+
+    const { result } = renderHook(() => useFocusMode());
+
+    expect(result.current.focus).toBe(true);
+  });
+});