fix: CDN优选IP支持IPv6、格式校验及错误降级

- replaceHost() 自动为裸 IPv6 地址包裹方括号，修复 IPv6 URL 畸形问题
- 新增 ValidateServerIPs() 校验 IP 格式，保存配置时拦截非法输入
- GetServerIPList() 运行时自动过滤非法 IP 条目
- CDN IP 尝试时 TLS/认证错误降级为非致命，跳过该 IP 继续故障转移
  而非终止整个连接循环（仅原始域名 ipIndex==0 时才判为致命）
- 占位符提示支持 IPv4/IPv6

internal/i18n/en.toml

@@ -45,6 +45,7 @@ DlgDeleteProfileMsg = 'Delete profile "{{.name}}" and its stored credentials?'
 DlgProfileTitle = "Profile"
 DlgValidationTitle = "Validation"
 DlgValidationMsg = "Name, Host, and Username are required."
+DlgInvalidIPMsg = "Invalid IP address(es): {{.ips}}"
 DlgDaemonError = "Daemon Error"
 DlgCredentialError = "Credential Error"
 DlgCredentialErrorMsg = "No password stored for this profile. Edit the profile to set it."
@@ -93,7 +94,7 @@ FieldMTUOverride = "MTU Override"
 PlaceholderCIDRs = "10.0.0.0/8, 172.16.0.0/12"
 PlaceholderMTU = "0 = use server MTU"
 PlaceholderPasswordUnchanged = "(unchanged)"
-PlaceholderServerIPs = "e.g. 1.2.3.4, 5.6.7.8"
+PlaceholderServerIPs = "IPv4/IPv6 supported, e.g. 1.2.3.4, 5.6.7.8"
 
 AuthModeBoth = "Both (JWT + Password)"
 AuthModeJWT = "JWT"