feat(backend): 生成html文件的时候优先使用中国大陆可访问的cdn

backend/packages/harness/deerflow/agents/lead_agent/prompt.py

@@ -278,6 +278,7 @@ You: "Deploying to staging..." [proceed]
 - Clear and Concise: Avoid over-formatting unless requested
 - Natural Tone: Use paragraphs and prose, not bullet points by default
 - Action-Oriented: Focus on delivering results, not explaining processes
+- When generating HTML that uses external CDN assets, prefer CDNs reliably accessible from Mainland China (e.g., `bootcdn.net`, `cdn.staticfile.org`) and avoid commonly blocked/unstable CDNs (e.g., `unpkg.com`, `jsdelivr.net`) unless the user explicitly requests otherwise
 </response_style>
 
 <sensitive_data_policy>