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/auth_handler_async_test.go

@@ -16,6 +16,7 @@ import (
 	"github.com/gofiber/fiber/v2"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/mock"
+	"gorm.io/gorm"
 )
 
 // --- Async Test Mocks ---
@@ -133,6 +134,10 @@ func (m *AsyncMockUserRepo) CountByCompanyCodes(ctx context.Context, codes []str
 	return nil, nil
 }
 
+func (m *AsyncMockUserRepo) DB() *gorm.DB {
+	return nil
+}
+
 func (m *AsyncMockUserRepo) UpdateUserLoginIDs(ctx context.Context, userID string, loginIDs []domain.UserLoginID) error {
 	return nil
 }