feat(proxy): 统一代理配置并支持 SOCKS5H 协议

- 新增 proxyutil 包，统一 HTTP/HTTPS/SOCKS5/SOCKS5H 代理配置逻辑
- SOCKS5H 支持服务端 DNS 解析，避免本地 DNS 泄露
- 移除 ProxyStrict 宽松模式，代理失败直接返回错误不回退直连
- 前端代理管理页面支持 SOCKS5H 协议的添加/编辑/批量导入
- 补充 IPv6 地址和特殊字符密码的边界测试

backend/internal/handler/admin/proxy_handler.go

@@ -26,7 +26,7 @@ func NewProxyHandler(adminService service.AdminService) *ProxyHandler {
 // CreateProxyRequest represents create proxy request
 type CreateProxyRequest struct {
 	Name     string `json:"name" binding:"required"`
-	Protocol string `json:"protocol" binding:"required,oneof=http https socks5"`
+	Protocol string `json:"protocol" binding:"required,oneof=http https socks5 socks5h"`
 	Host     string `json:"host" binding:"required"`
 	Port     int    `json:"port" binding:"required,min=1,max=65535"`
 	Username string `json:"username"`
@@ -36,7 +36,7 @@ type CreateProxyRequest struct {
 // UpdateProxyRequest represents update proxy request
 type UpdateProxyRequest struct {
 	Name     string `json:"name"`
-	Protocol string `json:"protocol" binding:"omitempty,oneof=http https socks5"`
+	Protocol string `json:"protocol" binding:"omitempty,oneof=http https socks5 socks5h"`
 	Host     string `json:"host"`
 	Port     int    `json:"port" binding:"omitempty,min=1,max=65535"`
 	Username string `json:"username"`
@@ -255,7 +255,7 @@ func (h *ProxyHandler) GetProxyAccounts(c *gin.Context) {
 
 // BatchCreateProxyItem represents a single proxy in batch create request
 type BatchCreateProxyItem struct {
-	Protocol string `json:"protocol" binding:"required,oneof=http https socks5"`
+	Protocol string `json:"protocol" binding:"required,oneof=http https socks5 socks5h"`
 	Host     string `json:"host" binding:"required"`
 	Port     int    `json:"port" binding:"required,min=1,max=65535"`
 	Username string `json:"username"`