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/handlerregression/dev_handler_headless_secret_test.go

@@ -9,6 +9,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"maps"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -196,14 +197,10 @@ func TestGetClient_ClientSecretFallbackPaths(t *testing.T) {
 			})
 
 			secretRepo := &mockSecretRepo{secrets: map[string]string{}}
-			for k, v := range tt.initialSecrets {
-				secretRepo.secrets[k] = v
-			}
+			maps.Copy(secretRepo.secrets, tt.initialSecrets)
 
 			redisRepo := &mockRedisRepo{data: map[string]string{}}
-			for k, v := range tt.initialRedis {
-				redisRepo.data[k] = v
-			}
+			maps.Copy(redisRepo.data, tt.initialRedis)
 
 			h := &handler.DevHandler{
 				Hydra: &service.HydraAdminService{