fix(conversations): always show group chats regardless of message count

Update conversation list queries to distinguish between private and group chats:
private conversations are only shown when they have messages (last_seq > 0), while group conversations are always displayed even without messages. This applies to both the paginated GetConversations method and the cursor-based GetConversationsByCursor method.

internal/repository/message_repo.go

@@ -158,10 +158,10 @@ func (r *messageRepository) GetConversations(userID string, page, pageSize int)
 	var convs []*model.Conversation
 	var total int64
 
-	// 获取总数（仅统计有消息的会话）
+	// 获取总数（私聊仅统计有消息的会话，群聊始终显示）
 	r.db.Model(&model.ConversationParticipant{}).
 		Joins("INNER JOIN conversations c ON c.id = conversation_participants.conversation_id").
-		Where("conversation_participants.user_id = ? AND conversation_participants.hidden_at IS NULL AND c.last_seq > 0", userID).
+		Where("conversation_participants.user_id = ? AND conversation_participants.hidden_at IS NULL AND (c.type != ? OR c.last_seq > 0)", userID, model.ConversationTypePrivate).
 		Count(&total)
 
 	if total == 0 {
@@ -171,7 +171,7 @@ func (r *messageRepository) GetConversations(userID string, page, pageSize int)
 	offset := (page - 1) * pageSize
 	err := r.db.Model(&model.Conversation{}).
 		Joins("INNER JOIN conversation_participants cp ON conversations.id = cp.conversation_id").
-		Where("cp.user_id = ? AND cp.hidden_at IS NULL AND conversations.last_seq > 0", userID).
+		Where("cp.user_id = ? AND cp.hidden_at IS NULL AND (conversations.type != ? OR conversations.last_seq > 0)", userID, model.ConversationTypePrivate).
 		Preload("Group").
 		Offset(offset).
 		Limit(pageSize).
@@ -798,7 +798,7 @@ func (r *messageRepository) GetConversationsByCursor(ctx context.Context, userID
 	// 构建基础查询 - 关联 conversation_participants 表
 	query := r.db.WithContext(ctx).Model(&model.Conversation{}).
 		Joins("INNER JOIN conversation_participants cp ON conversations.id = cp.conversation_id").
-		Where("cp.user_id = ? AND cp.hidden_at IS NULL AND conversations.last_seq > 0", userID).
+		Where("cp.user_id = ? AND cp.hidden_at IS NULL AND (conversations.type != ? OR conversations.last_seq > 0)", userID, model.ConversationTypePrivate).
 		Preload("Group")
 
 	// 会话列表需要特殊处理：先按置顶排序，再按更新时间排序