fix: remove undefined chapterIndex reference in Step4

src/app/features/novel-writer/components/pipeline/Step4ContentEditor.tsx

@@ -583,7 +583,7 @@ ${antiText}
       );
 
       const marathonSection = marathonOutline
-        ? `## 马拉松大纲（第${chapterIndex}章在整体弧中的位置）\n${marathonOutline}`
+        ? `## 马拉松大纲（当前章节在整体弧中的位置）\n${marathonOutline}`
         : '';
 
       const prompt = await buildGenerationPrompt(