feat: restore explicit loginId field and add to userfront signup flow

- Revert the removal of loginId from adminfront and backend.
- Prevent phone normalization logic from mangling custom employee ID login fields.
- Add an explicit 'loginId' optional input field to the userfront signup UI.
- Update AuthProxyService.signup and backend AuthHandler.Signup to transmit and map the 'loginId' parameter properly.

backend/internal/handler/auth_handler.go

@@ -451,8 +451,23 @@ func (h *AuthHandler) Signup(c *fiber.Ctx) error {
 		// grade는 기존 스키마 필수 키이므로 기본값을 설정
 		"grade": "member",
 	}
+
+	if req.LoginID != "" {
+		attributes["id"] = req.LoginID
+	}
+
+	// Sync custom field to LoginID if configured
+	if tenantID != nil && h.TenantService != nil {
+		if tenant, err := h.TenantService.GetTenant(c.Context(), *tenantID); err == nil && tenant != nil {
+			if loginIdField, ok := tenant.Config["loginIdField"].(string); ok && loginIdField != "" {
+				syncLoginID(attributes, req.Metadata, *tenantID, loginIdField)
+			}
+		}
+	}
+
 	brokerUser := &domain.BrokerUser{
 		Email:       req.Email,
+		LoginID:     extractTraitString(attributes, "id"),
 		Name:        req.Name,
 		PhoneNumber: normalizedPhone,
 		Attributes:  attributes,