feat: add channel tabs and chat support

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

@@ -0,0 +1,16 @@
+package com.openisle.dto;
+
+import lombok.Getter;
+import lombok.Setter;
+
+@Getter
+@Setter
+public class ChannelDto {
+    private Long id;
+    private String name;
+    private String description;
+    private String avatar;
+    private Long conversationId;
+    private int memberCount;
+    private long unreadCount;
+}