feat(foreshadow): 写作台本章伏笔主动提醒（前端）

- 新增 chapterForeshadow 纯分类：按当前章分 可回收/待回收/已逾期（启用闲置的 isWindowApproaching）
- useChapterForeshadow hook（复用 GET /foreshadow，切章仅重分类不重拉）
- 右栏「本章参考」+ ContextDrawer 伏笔页新增「本章伏笔」实时清单，
  分色展示 + 跳看板链接 + 空态；标注『按已验收进度』

apps/web/lib/foreshadow/useChapterForeshadow.test.ts

@@ -0,0 +1,79 @@
+// @vitest-environment jsdom
+import { renderHook, waitFor } from "@testing-library/react";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+import type { ForeshadowView } from "@/lib/api/types";
+import { useChapterForeshadow } from "./useChapterForeshadow";
+
+// 后端客户端是 hook 的外部副作用边界，单测一律 mock。
+const get = vi.fn();
+vi.mock("@/lib/api/client", () => ({
+  api: { GET: (...a: unknown[]) => get(...a) },
+}));
+
+function makeRow(over: Partial<ForeshadowView>): ForeshadowView {
+  return { code: "F-001", title: "线索", status: "OPEN", ...over } as ForeshadowView;
+}
+
+describe("useChapterForeshadow", () => {
+  beforeEach(() => get.mockReset());
+  afterEach(() => vi.clearAllMocks());
+
+  it("拉取成功后按当前章号分类", async () => {
+    get.mockResolvedValue({
+      data: {
+        foreshadow: [
+          makeRow({ code: "R", status: "OPEN", expected_close_from: 40, expected_close_to: 60 }),
+          makeRow({ code: "P", status: "OPEN", expected_close_from: 80 }),
+          makeRow({ code: "O", status: "OVERDUE" }),
+        ],
+      },
+      error: null,
+    });
+
+    const { result } = renderHook(() => useChapterForeshadow("p1", 50));
+
+    await waitFor(() => expect(result.current.status).toBe("ready"));
+    expect(result.current.groups.recyclable.map((v) => v.code)).toEqual(["R"]);
+    expect(result.current.groups.pending.map((v) => v.code)).toEqual(["P"]);
+    expect(result.current.groups.overdue.map((v) => v.code)).toEqual(["O"]);
+  });
+
+  it("拉取失败 → status=error、空分组", async () => {
+    get.mockResolvedValue({ data: null, error: { error: {} } });
+
+    const { result } = renderHook(() => useChapterForeshadow("p1", 3));
+
+    await waitFor(() => expect(result.current.status).toBe("error"));
+    expect(result.current.groups.recyclable).toEqual([]);
+    expect(result.current.groups.pending).toEqual([]);
+    expect(result.current.groups.overdue).toEqual([]);
+  });
+
+  it("切章不重新请求，仅重新分类", async () => {
+    get.mockResolvedValue({
+      data: {
+        foreshadow: [
+          makeRow({ code: "W", status: "OPEN", expected_close_from: 40, expected_close_to: 60 }),
+        ],
+      },
+      error: null,
+    });
+
+    const { result, rerender } = renderHook(
+      ({ ch }: { ch: number }) => useChapterForeshadow("p1", ch),
+      { initialProps: { ch: 50 } },
+    );
+
+    await waitFor(() => expect(result.current.status).toBe("ready"));
+    expect(result.current.groups.recyclable.map((v) => v.code)).toEqual(["W"]);
+
+    // 切到窗口外的章：仍是同一份数据，重新分类进 pending，且未再次 GET。
+    rerender({ ch: 10 });
+    await waitFor(() =>
+      expect(result.current.groups.pending.map((v) => v.code)).toEqual(["W"]),
+    );
+    expect(result.current.groups.recyclable).toEqual([]);
+    expect(get).toHaveBeenCalledTimes(1);
+  });
+});