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_signup_test.go

@@ -92,7 +92,7 @@ func TestSignup_TenantSlugValidation(t *testing.T) {
 	app.Post("/signup", h.Signup)
 
 	// Prepare mock state (already verified email/phone)
-	verifiedState, _ := json.Marshal(map[string]interface{}{
+	verifiedState, _ := json.Marshal(map[string]any{
 		"verified":   true,
 		"expires_at": time.Now().Add(time.Hour).Unix(),
 	})