Use DTO for activity list

backend/src/main/java/com/openisle/dto/ActivityDto.java

@@ -0,0 +1,21 @@
+package com.openisle.dto;
+
+import com.openisle.model.ActivityType;
+import lombok.Data;
+
+import java.time.LocalDateTime;
+
+/**
+ * DTO representing an activity without participant details.
+ */
+@Data
+public class ActivityDto {
+    private Long id;
+    private String title;
+    private String icon;
+    private String content;
+    private LocalDateTime startTime;
+    private LocalDateTime endTime;
+    private ActivityType type;
+    private boolean ended;
+}