scene：创建用户认证场景和UI资源

- 添加认证场景 auth_scene.tscn，包含登录和注册界面
- 添加认证相关的UI背景图片和框架素材
- 实现响应式布局，支持登录/注册界面切换
- 包含完整的表单输入控件和验证码输入区域

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