fix(openai): Mobile RT 账号隐私设置失败

1. CreateAccount 补齐 OpenAI OAuth 隐私入口（与 BatchCreate 对齐）
2. disableOpenAITraining 请求头修正：覆盖 ImpersonateChrome() 的
   浏览器导航默认头（accept: text/html, sec-fetch-mode: navigate），
   改为 API 请求语义（Accept: application/json, sec-fetch-mode: cors），
   避免 Cloudflare 将 PATCH API 请求误判为异常导航流量而拦截

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/service/openai_privacy_service.go

@@ -56,6 +56,10 @@ func disableOpenAITraining(ctx context.Context, clientFactory PrivacyClientFacto
 		SetHeader("Authorization", "Bearer "+accessToken).
 		SetHeader("Origin", "https://chatgpt.com").
 		SetHeader("Referer", "https://chatgpt.com/").
+		SetHeader("Accept", "application/json").
+		SetHeader("sec-fetch-mode", "cors").
+		SetHeader("sec-fetch-site", "same-origin").
+		SetHeader("sec-fetch-dest", "empty").
 		SetQueryParam("feature", "training_allowed").
 		SetQueryParam("value", "false").
 		Patch(openAISettingsURL)