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/tenant_handler.go

@@ -13,6 +13,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"maps"
 	"sort"
 	"strings"
 	"time"
@@ -312,10 +313,7 @@ func (h *TenantHandler) ListTenants(c *fiber.Ctx) error {
 			}
 			offset = 0
 		} else if offset < len(tenants) {
-			end := offset + limit
-			if end > len(tenants) {
-				end = len(tenants)
-			}
+			end := min(offset+limit, len(tenants))
 			tenants = tenants[offset:end]
 			if total > int64(end) && len(tenants) > 0 {
 				last := tenants[len(tenants)-1]
@@ -980,12 +978,8 @@ func mergeTenantCSVRecordConfig(current domain.JSONMap, record tenantCSVRecord)
 	}
 
 	merged := make(domain.JSONMap, len(current)+len(recordConfig))
-	for key, value := range current {
-		merged[key] = value
-	}
-	for key, value := range recordConfig {
-		merged[key] = value
-	}
+	maps.Copy(merged, current)
+	maps.Copy(merged, recordConfig)
 	return merged, true, nil
 }