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`,

apps/web/lib/nav/items.ts

@@ -46,6 +46,43 @@ export function projectNavItems(projectId: string): NavItem[] {
   ];
 }
 
+// 左栏一个分组：可选小标题 + 若干条目（P2-1）。全局组 label=null（顶部直显、无小标题）；
+// 项目组复用现有"本作"式小标题样式。分组只重排 + 加标题，链接/路由/激活键均不变（行为保持）。
+export interface NavSection {
+  label: string | null;
+  items: readonly NavItem[];
+}
+
+// 项目级入口的语义分组（P2-1）：把 10 个平铺项归为 写作/资料库/审阅/更多，便于扫读。
+// 工具箱归入「更多」——原分组清单只列了 规则·技能库，收进"更多"以不丢任何入口
+// （见 memory/decisions.md 2026-07-10 条）。
+const PROJECT_GROUPS: readonly { label: string; keys: readonly ActiveNav[] }[] =
+  [
+    { label: "写作", keys: ["write"] },
+    { label: "资料库", keys: ["outline", "codex", "foreshadow", "style"] },
+    { label: "审阅", keys: ["review", "chains"] },
+    { label: "更多", keys: ["toolbox", "rules", "skills"] },
+  ];
+
+// 左导航分组（桌面侧栏 / 移动抽屉共用）：全局组恒在顶部，项目内页再展开四个项目组。
+export function navSections(projectId?: string): NavSection[] {
+  const global: NavSection = { label: null, items: GLOBAL_NAV_ITEMS };
+  if (projectId === undefined) return [global];
+
+  const byKey = new Map(projectNavItems(projectId).map((item) => [item.key, item]));
+  const projectSections = PROJECT_GROUPS.map((group) => ({
+    label: group.label,
+    items: group.keys.map((key) => {
+      const item = byKey.get(key);
+      if (item === undefined) {
+        throw new Error(`navSections: 缺少项目入口 ${key}`);
+      }
+      return item;
+    }),
+  }));
+  return [global, ...projectSections];
+}
+
 export interface NavActiveContext {
   pathname: string;
   activeNav?: ActiveNav;