feat: add OpenClaw User-Agent toggle, default off

Add a switch in the OpenClaw provider form to optionally send a browser
User-Agent header. The toggle defaults to off — only providers that
explicitly include headers in their preset or config will have it enabled.

Remove the previous auto-injection logic that force-added User-Agent on
every preset load and new provider creation.

src/types.ts

@@ -522,6 +522,7 @@ export interface OpenClawProviderConfig {
   apiKey?: string; // API 密钥
   api?: string; // API 协议类型（如 "openai-completions"、"anthropic"）
   models?: OpenClawModel[]; // 可用模型列表
+  headers?: Record<string, string>; // 自定义请求头（如 User-Agent）
 }
 
 // OpenClaw agents.defaults 完整配置