feat: improve ai writing imitation flow

frontend-web/src/open-canvas/api/ai-writing-sample.ts

@@ -17,6 +17,12 @@ export async function extractAIWritingSample(file: File): Promise<AIWritingSampl
   });
   if (!resp.ok) {
     const text = await resp.text().catch(() => '');
+    let detail = '';
+    try {
+      const parsed = text ? JSON.parse(text) as { detail?: unknown } : null;
+      detail = parsed?.detail ? String(parsed.detail) : '';
+    } catch {}
+    if (detail) throw new Error(detail);
     throw new Error(text || `样文解析失败：${resp.status}`);
   }
   return resp.json();