feat(frontend): 「目录」列可折叠、记忆折叠状态（UX P2-2）

useChapterListCollapse 用 localStorage 记忆折叠态（SSR 首帧恒展开避免 hydration 不一致）；收起降为带展开按钮的窄轨、让宽给正文，展开时标题旁加收起按钮；三栏栅格据折叠态切列宽，章节列表/高亮/切章导航全保留。

apps/web/components/workbench/Workbench.tsx

@@ -59,6 +59,7 @@ import { GenrePicker } from "./GenrePicker";
 import { GENRES } from "@/lib/wizard/wizard";
 import { useGenreGate, toGenreDirective } from "@/lib/workbench/useGenreGate";
 import { useAiConversation } from "@/lib/workbench/useAiConversation";
+import { useChapterListCollapse } from "@/lib/workbench/useChapterListCollapse";
 
 interface WorkbenchProps {
   project: ProjectResponse;
@@ -104,6 +105,8 @@ export function Workbench({
   // AC-3 AI 对话抽屉：Workbench 级单实例（append 即使抽屉关着也能触发）；flag 灰度。
   const [conversationOpen, setConversationOpen] = useState(false);
   const conversation = useAiConversation(project.id, chapterNo);
+  // P2-2：桌面「目录」列可收起（记忆状态）；收起时把宽让给正文，窄轨留一个展开入口。
+  const chapterList = useChapterListCollapse();
   const conversationTriggerRef = useRef<HTMLButtonElement>(null);
   const toast = useToast();
   const autosave = useAutosave(project.id, chapterNo, initialText);
@@ -292,11 +295,19 @@ export function Workbench({
       projectId={project.id}
       activeNav="write"
     >
-      <div className="grid min-h-[calc(100vh-var(--chrome,4rem))] grid-cols-1 xl:h-[calc(100vh-var(--chrome,4rem))] xl:grid-cols-[12rem_1fr_18rem]">
+      <div
+        className={`grid min-h-[calc(100vh-var(--chrome,4rem))] grid-cols-1 xl:h-[calc(100vh-var(--chrome,4rem))] ${
+          chapterList.collapsed
+            ? "xl:grid-cols-[2.75rem_1fr_18rem]"
+            : "xl:grid-cols-[12rem_1fr_18rem]"
+        }`}
+      >
         <ChapterList
           projectId={project.id}
           chapters={chapters}
           currentChapterNo={chapterNo}
+          collapsed={chapterList.collapsed}
+          onToggle={chapterList.toggle}
         />
 
         <section className="flex min-w-0 flex-col bg-bg">