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/utils/masking.go

@@ -33,7 +33,7 @@ func MaskSensitiveJSON(data []byte) []byte {
 		return data
 	}
 
-	var obj interface{}
+	var obj any
 	if err := json.Unmarshal(data, &obj); err != nil {
 		// Not a JSON object/array, return as is
 		return data
@@ -48,10 +48,10 @@ func MaskSensitiveJSON(data []byte) []byte {
 	return result
 }
 
-func maskValue(v interface{}) interface{} {
+func maskValue(v any) any {
 	switch val := v.(type) {
-	case map[string]interface{}:
-		newMap := make(map[string]interface{}, len(val))
+	case map[string]any:
+		newMap := make(map[string]any, len(val))
 		for k, v := range val {
 			if isSensitive(k) {
 				newMap[k] = "*****"
@@ -60,8 +60,8 @@ func maskValue(v interface{}) interface{} {
 			}
 		}
 		return newMap
-	case []interface{}:
-		newArr := make([]interface{}, len(val))
+	case []any:
+		newArr := make([]any, len(val))
 		for i, v := range val {
 			newArr[i] = maskValue(v)
 		}