feat(user): support fixed UUID registration and enhance bulk import results

- Added support for fixed UUIDs during bulk registration (Search-first + ExternalID mapping)
- Implemented idempotency and visibility restoration for soft-deleted users
- Enhanced bulk upload UI to show 'New/Updated/Unchanged' status and modified fields
- Added logic to reclaim identifiers (login_id) from colliding records
- Added frontend E2E and backend unit tests for UUID integrity and conflict handling
- Fixed i18n, formatting, and mock tests to satisfy code-check
- Applied 'go fix' for 'omitzero' tags and general Go standards

backend/internal/domain/user.go

@@ -2,6 +2,7 @@ package domain
 
 import (
 	"fmt"
+	"slices"
 	"strings"
 	"time"
 
@@ -208,10 +209,8 @@ func ValidateLoginID(loginID string, emails []string, phone string) error {
 
 	reserved := []string{"admin", "system", "root", "master", "superuser", "guest", "operator"}
 	lowerID := strings.ToLower(loginID)
-	for _, r := range reserved {
-		if lowerID == r {
-			return fmt.Errorf("reserved ID cannot be used")
-		}
+	if slices.Contains(reserved, lowerID) {
+		return fmt.Errorf("reserved ID cannot be used")
 	}
 
 	return nil