feat(auth): 注册服务协议 - 名称与条款改为后台可配置并同步注册页展示

后端：
- RegistrationSettingRequest 协议字段改为可选指针，注册开关单独保存不再覆盖协议正文
- 新增 RegistrationAgreement / AgreementTitleForMessage：标题未配置时跟随品牌名生成，读取失败返回空标题而非猜一个不存在的协议名
- PublicAuthSettings 下发 agreementTitle / agreementContent，首用户分支同样携带
- 新增 registration_agreement_test.go 覆盖品牌名兜底、存储合并与失败降级

前端：
- 注册页协议标题与条款改为消费后台配置；读取失败时隐藏协议入口并支持重新读取
- 验证码获取按钮内联到输入框右侧，联系方式为空即禁用；sms_email 双通道合并为单输入框
- 协议弹窗重排：条目标题独立成行加重、空态虚线提示、底部补充同意说明；深浅主题适配
- 协议勾选行改为状态化区域，复选框适配 AntD 6 结构（方框在 .ant-checkbox 本体，修正深色主题白底白勾）
- 后台「服务协议名称与条款」区块补齐表单内边距并对齐区块头/页脚，协议名称限宽

文档：http-api.mdx 更新协议字段语义与失败降级说明；pending-test.mdx 增加手工验证清单

web/src/services/api/auth.ts

@@ -378,7 +378,7 @@ export type RuntimePolicySetting = {
 };
 
 export function getAuthSettings() {
-    return http.get<VerificationPolicy & { firstUser: boolean; registrationEnabled: boolean; linuxdoEnabled: boolean; emailEnabled: boolean; emailCodeRequired: boolean; smsBindingAvailable: boolean; emailBindingAvailable: boolean }>("/auth/settings");
+    return http.get<VerificationPolicy & { firstUser: boolean; registrationEnabled: boolean; linuxdoEnabled: boolean; emailEnabled: boolean; emailCodeRequired: boolean; smsBindingAvailable: boolean; emailBindingAvailable: boolean; agreementTitle?: string; agreementContent?: string }>("/auth/settings");
 }
 
 export function linuxDOLoginURL(next: string, acceptedTerms?: boolean) {