fix compilation

app/service/Ld56MasterServer.scala

@@ -6,6 +6,7 @@ import play.api.libs.json.*
 import java.net.{InetSocketAddress, SocketAddress}
 import java.nio.ByteBuffer
 import java.nio.channels.DatagramChannel
+import java.nio.charset.StandardCharsets
 import java.time.{Duration, Instant}
 import java.util.concurrent.ConcurrentHashMap
 import scala.jdk.CollectionConverters.CollectionHasAsScala