feat(ux): T4-a 全局 AI 工具条 — 项目页顶部常驻写本章/审稿/大纲/设定库/工具箱

新 lib/nav/ai-tools.ts (aiToolItems) + components/AiToolbar.tsx 服务端组件；AppShell 顶栏下渲染（仅项目页），用 --chrome CSS 变量统一 chrome 高度，6 个固定高度页改用 calc(100vh-var(--chrome,4rem)) 适配。TDD: ai-tools 测。前端门禁绿: lint/tsc/vitest/build。

apps/web/components/AppShell.tsx

@@ -2,6 +2,7 @@ import Link from "next/link";
 import type { ReactNode } from "react";
 
 import type { ActiveNav } from "@/lib/nav/items";
+import { AiToolbar } from "./AiToolbar";
 import { LeftNav } from "./LeftNav";
 import { NavDrawer } from "./NavDrawer";
 
@@ -25,7 +26,10 @@ export function AppShell({
   activeNav,
 }: AppShellProps) {
   return (
-    <div className="min-h-screen">
+    <div
+      className="min-h-screen"
+      style={{ ["--chrome" as string]: projectId ? "7rem" : "4rem" }}
+    >
       <header className="flex h-16 items-center gap-4 border-b border-line bg-panel px-4 sm:px-6">
         <NavDrawer projectId={projectId} activeNav={activeNav} />
         <Link
@@ -50,9 +54,14 @@ export function AppShell({
           </Link>
         </nav>
       </header>
+      {projectId ? (
+        <AiToolbar projectId={projectId} activeNav={activeNav} />
+      ) : null}
       <div className="flex">
         <LeftNav projectId={projectId} activeNav={activeNav} />
-        <main className="min-h-[calc(100vh-4rem)] flex-1 bg-bg">{children}</main>
+        <main className="min-h-[calc(100vh-var(--chrome,4rem))] flex-1 bg-bg">
+          {children}
+        </main>
       </div>
     </div>
   );