edit the interface IQuizService & create the client call back

src/main/java/org/example/shared/IClientCallback.java

+package org.example.shared;
+
+import java.rmi.Remote;
+import java.rmi.RemoteException;
+
+public interface IClientCallback extends Remote {
+    void onLeaderboardUpdate(String leaderboard) throws RemoteException;
+}
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src/main/java/org/example/shared/IQuizService.java

@@ -5,12 +5,9 @@ import java.rmi.RemoteException;
 
 
 public interface IQuizService extends Remote {
+    String getRandomQuestion() throws RemoteException;
+    String submitAnswer(String studentName, String answer) throws RemoteException;
 
-    public String getRandomQuestion() throws RemoteException;
-
-    public String submitAnswer(String studentName, String answer) throws RemoteException;
-
-    public String getLeaderboard() throws RemoteException;
-
-    public String authenticateStudent(String studentName) throws RemoteException;
+    // Combined authentication + callback registration
+    String authenticateStudent(String studentName, IClientCallback callback) throws RemoteException;
 }
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