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/hanmac_email.go

@@ -98,15 +98,15 @@ var hanmacInitialRomanization = []string{
 
 func SplitEmailDomain(email string) (string, string, error) {
 	normalized := strings.ToLower(strings.TrimSpace(email))
-	at := strings.Index(normalized, "@")
-	if at < 0 {
+	before, after, ok := strings.Cut(normalized, "@")
+	if !ok {
 		return "", "", errors.New("email must contain @")
 	}
 	if strings.Count(normalized, "@") != 1 {
 		return "", "", errors.New("email must contain one @")
 	}
-	localPart := strings.TrimSpace(normalized[:at])
-	domainPart := strings.TrimSpace(normalized[at+1:])
+	localPart := strings.TrimSpace(before)
+	domainPart := strings.TrimSpace(after)
 	if domainPart == "" || !strings.Contains(domainPart, ".") {
 		return "", "", errors.New("email domain is invalid")
 	}