feat(frontend): 进来即写——落地页「直接开始写」直达空白编辑器，敲字后延迟落库

Phase 0（写作工作台重构）：新增 /write 草稿编辑器入口，只有敲入实质正文并停顿后
才懒建占位「未命名草稿」项目并种入首章草稿（useDeferredDraft，双重幂等闸 + 草稿落库
非致命降级），随即 replace 换入完整工作台；空进空出不落库，杜绝孤儿草稿。
落地页 CTA 改为「直接开始写」为主、「先立项」（立项向导）为辅。

apps/web/lib/start/useDeferredDraft.test.ts

@@ -0,0 +1,134 @@
+// @vitest-environment jsdom
+import { act, renderHook } from "@testing-library/react";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+import {
+  DRAFT_FIRST_CHAPTER,
+  DRAFT_PLACEHOLDER_TITLE,
+  useDeferredDraft,
+} from "./useDeferredDraft";
+
+// 后端客户端与 Toast 是 hook 的外部副作用边界，单测一律 mock。
+const post = vi.fn();
+const put = vi.fn();
+const toast = vi.fn();
+vi.mock("@/lib/api/client", () => ({
+  api: {
+    POST: (...a: unknown[]) => post(...a),
+    PUT: (...a: unknown[]) => put(...a),
+  },
+}));
+vi.mock("@/components/Toast", () => ({ useToast: () => toast }));
+
+describe("useDeferredDraft", () => {
+  beforeEach(() => {
+    post.mockReset();
+    put.mockReset();
+    toast.mockReset();
+  });
+  afterEach(() => vi.clearAllMocks());
+
+  it("初始为 idle", () => {
+    const { result } = renderHook(() => useDeferredDraft());
+    expect(result.current.status).toBe("idle");
+  });
+
+  it("首次落库：懒建占位项目 + 用已敲入正文种首章草稿，返回新项目 id", async () => {
+    post.mockResolvedValue({ data: { id: "p1" }, error: null });
+    put.mockResolvedValue({ data: {}, error: null });
+
+    const { result } = renderHook(() => useDeferredDraft());
+    let id: string | null = null;
+    await act(async () => {
+      id = await result.current.ensureCreated("第一段正文");
+    });
+
+    expect(id).toBe("p1");
+    expect(result.current.status).toBe("created");
+    expect(post).toHaveBeenCalledTimes(1);
+    expect(post).toHaveBeenCalledWith("/projects", {
+      body: { title: DRAFT_PLACEHOLDER_TITLE },
+    });
+    expect(put).toHaveBeenCalledWith(
+      "/projects/{project_id}/chapters/{chapter_no}/draft",
+      {
+        params: { path: { project_id: "p1", chapter_no: DRAFT_FIRST_CHAPTER } },
+        body: { text: "第一段正文" },
+      },
+    );
+    expect(toast).not.toHaveBeenCalled();
+  });
+
+  it("幂等：已建后再次调用复用同一 id，绝不重复 POST", async () => {
+    post.mockResolvedValue({ data: { id: "p1" }, error: null });
+    put.mockResolvedValue({ data: {}, error: null });
+
+    const { result } = renderHook(() => useDeferredDraft());
+    await act(async () => {
+      await result.current.ensureCreated("正文一");
+    });
+    let second: string | null = null;
+    await act(async () => {
+      second = await result.current.ensureCreated("正文二");
+    });
+
+    expect(second).toBe("p1");
+    expect(post).toHaveBeenCalledTimes(1);
+  });
+
+  it("正文为空白：仍懒建项目但跳过草稿落库", async () => {
+    post.mockResolvedValue({ data: { id: "p1" }, error: null });
+
+    const { result } = renderHook(() => useDeferredDraft());
+    await act(async () => {
+      await result.current.ensureCreated("   ");
+    });
+
+    expect(post).toHaveBeenCalledTimes(1);
+    expect(put).not.toHaveBeenCalled();
+  });
+
+  it("创建失败：status=error、弹错误 toast、返回 null、不落草稿", async () => {
+    post.mockResolvedValue({ data: null, error: { detail: "配额不足" } });
+
+    const { result } = renderHook(() => useDeferredDraft());
+    let id: string | null = "sentinel";
+    await act(async () => {
+      id = await result.current.ensureCreated("正文");
+    });
+
+    expect(id).toBeNull();
+    expect(result.current.status).toBe("error");
+    expect(put).not.toHaveBeenCalled();
+    expect(toast).toHaveBeenCalledWith(expect.any(String), "error");
+  });
+
+  it("草稿落库失败为非致命：项目已建仍返回 id，提示但不阻断", async () => {
+    post.mockResolvedValue({ data: { id: "p1" }, error: null });
+    put.mockResolvedValue({ data: null, error: { detail: "写库失败" } });
+
+    const { result } = renderHook(() => useDeferredDraft());
+    let id: string | null = null;
+    await act(async () => {
+      id = await result.current.ensureCreated("正文");
+    });
+
+    expect(id).toBe("p1");
+    expect(result.current.status).toBe("created");
+    expect(toast).toHaveBeenCalledWith(expect.any(String), "info");
+  });
+
+  it("请求抛异常：status=error、弹网络异常 toast、返回 null", async () => {
+    post.mockRejectedValue(new Error("network down"));
+
+    const { result } = renderHook(() => useDeferredDraft());
+    let id: string | null = "sentinel";
+    await act(async () => {
+      id = await result.current.ensureCreated("正文");
+    });
+
+    expect(id).toBeNull();
+    expect(result.current.status).toBe("error");
+    expect(toast).toHaveBeenCalledWith(expect.any(String), "error");
+  });
+});