feat(privacy): 为 OpenAI OAuth 账号添加前端手动设置隐私按钮

复用已有的 set-privacy API 端点，Handler 通过 platform 分发到
ForceOpenAIPrivacy / ForceAntigravityPrivacy，前端 AccountActionMenu
扩展隐私按钮支持 OpenAI OAuth 账号。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/internal/handler/admin/admin_service_stub_test.go

@@ -449,6 +449,10 @@ func (s *stubAdminService) EnsureAntigravityPrivacy(ctx context.Context, account
 	return ""
 }
 
+func (s *stubAdminService) ForceOpenAIPrivacy(ctx context.Context, account *service.Account) string {
+	return ""
+}
+
 func (s *stubAdminService) ForceAntigravityPrivacy(ctx context.Context, account *service.Account) string {
 	return ""
 }