Add core models, DTOs, and services for EchoHub chat application

- Created User, Channel, Message, and ServerInfo models with necessary properties.
- Added DTOs for user profiles, server information, and message handling.
- Implemented JWT authentication service for user login and token generation.
- Developed Entity Framework Core DbContext for database interactions.
- Introduced SignalR ChatHub for real-time messaging and presence tracking.
- Implemented file storage and image to ASCII conversion services.
- Set up CORS and middleware for API endpoints.
- Created launch settings and development configuration for server and web projects.

src/EchoHub.Core/Contracts/IEchoHubClient.cs

@@ -0,0 +1,16 @@
+using EchoHub.Core.DTOs;
+
+namespace EchoHub.Core.Contracts;
+
+/// <summary>
+/// Methods the server can invoke on connected clients.
+/// </summary>
+public interface IEchoHubClient
+{
+    Task ReceiveMessage(MessageDto message);
+    Task UserJoined(string channelName, string username);
+    Task UserLeft(string channelName, string username);
+    Task ChannelUpdated(ChannelDto channel);
+    Task UserStatusChanged(UserPresenceDto presence);
+    Task Error(string message);
+}