fix(backend): fix syncLoginID to allow fields named 'id' to be synced from custom schema

backend/internal/handler/user_handler.go

@@ -1209,7 +1209,6 @@ func (h *UserHandler) UpdateUser(c *fiber.Ctx) error {
 			}
 		}
 	}
-
 	finalLoginID := extractTraitString(traits, "id")
 	userEmail := extractTraitString(traits, "email")
 	userPhone := extractTraitString(traits, "phone")
@@ -1583,7 +1582,7 @@ func extractTraitString(traits map[string]interface{}, key string) string {
 
 // syncLoginID ensures that the 'id' trait (used as Kratos identifier) is in sync with the configured custom field.
 func syncLoginID(traits map[string]interface{}, metadata map[string]any, tenantID string, loginIDField string) {
-	if loginIDField == "" || loginIDField == "id" {
+	if loginIDField == "" {
 		return
 	}
 
@@ -1608,7 +1607,9 @@ func syncLoginID(traits map[string]interface{}, metadata map[string]any, tenantI
 	}
 
 	// 3. Check merged traits (which includes existing metadata)
-	if loginID == "" {
+	// Important: Skip this if loginIDField is "id" because traits["id"] is the TARGET, 
+	// and we don't want to sync "id" to "id" if we already checked metadata.
+	if loginID == "" && loginIDField != "id" {
 		// Existing trait (flat)
 		if val, ok := traits[loginIDField].(string); ok && val != "" {
 			loginID = val