feat(frontend): 左栏 10 项按写作/资料库/审阅/更多分组（UX P2-1）

navSections(projectId?) 纯函数把平铺项目项归为四组，NavItems 复用「本作」小标题模式逐组渲染；链接/路由/激活键不变，工具箱收进「更多」不丢入口。

apps/web/lib/nav/items.test.ts

@@ -3,6 +3,7 @@ import { describe, expect, it } from "vitest";
 import {
   GLOBAL_NAV_ITEMS,
   isNavItemActive,
+  navSections,
   projectNavItems,
   type NavItem,
 } from "./items";
@@ -32,6 +33,53 @@ describe("projectNavItems", () => {
   });
 });
 
+describe("navSections", () => {
+  it("returns only the unlabeled global section outside a project", () => {
+    const sections = navSections();
+
+    expect(sections).toHaveLength(1);
+    expect(sections[0]?.label).toBeNull();
+    expect(sections[0]?.items).toEqual(GLOBAL_NAV_ITEMS);
+  });
+
+  it("groups project entries under the four labeled sections (P2-1)", () => {
+    const sections = navSections(PID);
+
+    // 全局组（无标题）+ 写作 / 资料库 / 审阅 / 更多。
+    expect(sections.map((s) => s.label)).toEqual([
+      null,
+      "写作",
+      "资料库",
+      "审阅",
+      "更多",
+    ]);
+
+    const byLabel = (label: string): (string | undefined)[] =>
+      sections.find((s) => s.label === label)?.items.map((i) => i.key) ?? [];
+
+    expect(byLabel("写作")).toEqual(["write"]);
+    expect(byLabel("资料库")).toEqual([
+      "outline",
+      "codex",
+      "foreshadow",
+      "style",
+    ]);
+    expect(byLabel("审阅")).toEqual(["review", "chains"]);
+    expect(byLabel("更多")).toEqual(["toolbox", "rules", "skills"]);
+  });
+
+  it("places every project entry exactly once (no dropped or duplicated links)", () => {
+    const sections = navSections(PID);
+    const grouped = sections
+      .filter((s) => s.label !== null)
+      .flatMap((s) => s.items.map((i) => i.key));
+    const flat = projectNavItems(PID).map((i) => i.key);
+
+    expect([...grouped].sort()).toEqual([...flat].sort());
+    expect(grouped).toHaveLength(flat.length);
+  });
+});
+
 describe("isNavItemActive", () => {
   const write: NavItem = {
     href: `/projects/${PID}/write`,