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/service/relying_party_service_test.go

@@ -52,7 +52,7 @@ func TestRelyingPartyService_Create_Success(t *testing.T) {
 	tenantID := "tenant-1"
 	inputClient := domain.HydraClient{
 		ClientName: "Test App",
-		Metadata: map[string]interface{}{
+		Metadata: map[string]any{
 			"user_id": "creator-1",
 		},
 	}
@@ -127,7 +127,7 @@ func TestRelyingPartyService_Get_Success(t *testing.T) {
 		_ = json.NewEncoder(w).Encode(domain.HydraClient{
 			ClientID:   clientID,
 			ClientName: "Hydra Name",
-			Metadata: map[string]interface{}{
+			Metadata: map[string]any{
 				"tenant_id": "tenant-1",
 			},
 		})
@@ -180,7 +180,7 @@ func TestRelyingPartyService_Delete_Success(t *testing.T) {
 		if r.Method == http.MethodGet && strings.Contains(r.URL.Path, clientID) {
 			_ = json.NewEncoder(w).Encode(domain.HydraClient{
 				ClientID: clientID,
-				Metadata: map[string]interface{}{
+				Metadata: map[string]any{
 					"tenant_id": tenantID,
 					"user_id":   "creator-1",
 				},