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/sms_service.go

@@ -8,7 +8,7 @@ import (
 	"encoding/base64"
 	"encoding/json"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"log/slog"
 	"net/http"
 	"os"
@@ -82,7 +82,7 @@ func (s *SmsServiceImpl) SendSms(to, content string) error {
 	}
 	defer resp.Body.Close()
 
-	respBody, err := ioutil.ReadAll(resp.Body)
+	respBody, err := io.ReadAll(resp.Body)
 	if err != nil {
 		return fmt.Errorf("error reading response body: %w", err)
 	}