fix: address audit findings for notify, websearch and security

- Fix GetByKeyForAuth missing user.FieldEmail and user.FieldUsername (notifications sent to empty address)
- Guard against empty email in collectBalanceNotifyRecipients
- Remove non-atomic TotalRecharged read-modify-write in admin balance adjustment
- HTML-escape userName/siteName/accountName in notification email templates
- Fix timer leak in ProfileBalanceNotifyCard (add onUnmounted cleanup)
- Add warning log on websearch proxy URL resolution failure

backend/internal/service/websearch_config.go

@@ -235,6 +235,7 @@ func (s *SettingService) resolveProviderProxyURLs(ctx context.Context, cfg *WebS
 	}
 	proxies, err := s.proxyRepo.ListByIDs(ctx, ids)
 	if err != nil {
+		slog.Warn("websearch: failed to resolve proxy URLs", "error", err)
 		return nil
 	}
 	result := make(map[int64]string, len(proxies))