feat: add unread message count API

- Added `GET /messages/unread-count` endpoint to retrieve the count of unread messages for a user.
- Implemented `getUnreadCount` method in `MessagesService` and `MessagesRepository`.
- Updated frontend service to support fetching unread message count via API.

frontend/src/services/message.service.ts

@@ -29,6 +29,11 @@ export const MessageService = {
 		return data;
 	},
 
+	async getUnreadCount(): Promise<number> {
+		const { data } = await api.get<number>("/messages/unread-count");
+		return data;
+	},
+
 	async getMessages(conversationId: string): Promise<Message[]> {
 		const { data } = await api.get<Message[]>(
 			`/messages/conversations/${conversationId}`,