Add: Timestamp to User to store time of disconnect

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/user/User.java

@@ -1,17 +1,20 @@
 package ch.unibas.dmi.dbis.cs108.casono.server.domain.user;
 
 import ch.unibas.dmi.dbis.cs108.casono.server.network.sessions.SessionId;
+import java.time.Instant;
 import java.util.Optional;
 
 public class User {
     private final UserId id;
     private final String name;
     private SessionId sessionId;
+    private Instant disconnectedAt;
 
     public User(UserId id, String name, SessionId sessionId) {
         this.id = id;
         this.name = name;
         this.sessionId = sessionId;
+        this.disconnectedAt = null;
     }
 
     public UserId getId() {
@@ -25,4 +28,18 @@ public class User {
     public Optional<SessionId> getSessionId() {
         return Optional.ofNullable(sessionId);
     }
+
+    public Optional<Instant> getDisconnectedAt() {
+        return Optional.ofNullable(disconnectedAt);
+    }
+
+    public void reassignSession(SessionId sessionId) {
+        this.sessionId = sessionId;
+        this.disconnectedAt = null;
+    }
+
+    public void markDisconnected() {
+        this.sessionId = null;
+        this.disconnectedAt = Instant.now();
+    }
 }