refactor(frontend): 删顶部「AI 工具条」，--chrome 归位 4rem（写作台 UX P0-3）

顶栏「AI 工具条」5 项全是重复左栏的导航链接，且让「写本章」同时是导航
Link 与生成动作两种相反含义。删掉后导航只归左栏，同时消灭最严重的三重复
+ 假「AI」标签 + 「写本章」双重含义，并省下 48px（--chrome 7rem→4rem）。
死代码 AiToolbar/AiToolbarMoreMenu/ai-tools 全仓仅 AppShell 引用，一并移除。

apps/web/components/AppShell.tsx

@@ -4,7 +4,6 @@ import { Settings } from "lucide-react";
 
 import type { ActiveNav } from "@/lib/nav/items";
 import { buttonClass, focusRing } from "@/lib/ui/variants";
-import { AiToolbar } from "./AiToolbar";
 import { CommandSearchButton } from "./command/CommandPaletteMount";
 import { LeftNav } from "./LeftNav";
 import { NavDrawer } from "./NavDrawer";
@@ -32,7 +31,7 @@ export function AppShell({
   return (
     <div
       className="min-h-screen"
-      style={{ ["--chrome" as string]: projectId ? "7rem" : "4rem" }}
+      style={{ ["--chrome" as string]: "4rem" }}
     >
       <a
         href="#main"
@@ -80,9 +79,6 @@ export function AppShell({
           </Link>
         </nav>
       </header>
-      {projectId ? (
-        <AiToolbar projectId={projectId} activeNav={activeNav} />
-      ) : null}
       <div className="flex">
         <LeftNav projectId={projectId} activeNav={activeNav} />
         <main