feat(admin): 代理密码可见性 + 复制代理 URL 功能

- 新增 AdminProxy / AdminProxyWithAccountCount DTO，遵循项目 Admin DTO 分层模式
- Proxy.Password 恢复 json:"-" 隐藏，ProxyFromService 不再赋值密码（纵深防御）
- 管理员接口使用 ProxyFromServiceAdmin / ProxyWithAccountCountFromServiceAdmin
- 前端代理列表新增 Auth 列：显示用户名 + 掩码密码 + 眼睛图标切换可见性
- Address 列新增复制按钮：左键复制完整 URL，右键选择格式
- 编辑模态框密码预填充 + 脏标记，避免误更新

backend/internal/handler/dto/mappers.go

@@ -293,7 +293,6 @@ func ProxyFromService(p *service.Proxy) *Proxy {
 		Host:      p.Host,
 		Port:      p.Port,
 		Username:  p.Username,
-		Password:  p.Password,
 		Status:    p.Status,
 		CreatedAt: p.CreatedAt,
 		UpdatedAt: p.UpdatedAt,
@@ -323,6 +322,51 @@ func ProxyWithAccountCountFromService(p *service.ProxyWithAccountCount) *ProxyWi
 	}
 }
 
+// ProxyFromServiceAdmin converts a service Proxy to AdminProxy DTO for admin users.
+// It includes the password field - user-facing endpoints must not use this.
+func ProxyFromServiceAdmin(p *service.Proxy) *AdminProxy {
+	if p == nil {
+		return nil
+	}
+	base := ProxyFromService(p)
+	if base == nil {
+		return nil
+	}
+	return &AdminProxy{
+		Proxy:    *base,
+		Password: p.Password,
+	}
+}
+
+// ProxyWithAccountCountFromServiceAdmin converts a service ProxyWithAccountCount to AdminProxyWithAccountCount DTO.
+// It includes the password field - user-facing endpoints must not use this.
+func ProxyWithAccountCountFromServiceAdmin(p *service.ProxyWithAccountCount) *AdminProxyWithAccountCount {
+	if p == nil {
+		return nil
+	}
+	admin := ProxyFromServiceAdmin(&p.Proxy)
+	if admin == nil {
+		return nil
+	}
+	return &AdminProxyWithAccountCount{
+		AdminProxy:     *admin,
+		AccountCount:   p.AccountCount,
+		LatencyMs:      p.LatencyMs,
+		LatencyStatus:  p.LatencyStatus,
+		LatencyMessage: p.LatencyMessage,
+		IPAddress:      p.IPAddress,
+		Country:        p.Country,
+		CountryCode:    p.CountryCode,
+		Region:         p.Region,
+		City:           p.City,
+		QualityStatus:  p.QualityStatus,
+		QualityScore:   p.QualityScore,
+		QualityGrade:   p.QualityGrade,
+		QualitySummary: p.QualitySummary,
+		QualityChecked: p.QualityChecked,
+	}
+}
+
 func ProxyAccountSummaryFromService(a *service.ProxyAccountSummary) *ProxyAccountSummary {
 	if a == nil {
 		return nil