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/handler/hanmac_email_policy.go

@@ -4,6 +4,7 @@ import (
 	"baron-sso-backend/internal/domain"
 	"context"
 	"fmt"
+	"slices"
 	"strings"
 )
 
@@ -235,10 +236,8 @@ func nextAvailableHanmacLocalPart(base string, usedLocalParts map[string]bool) s
 }
 
 func appendUniqueString(values []string, value string) []string {
-	for _, existing := range values {
-		if existing == value {
-			return values
-		}
+	if slices.Contains(values, value) {
+		return values
 	}
 	return append(values, value)
 }