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/Models/Message.cs

@@ -0,0 +1,17 @@
+namespace EchoHub.Core.Models;
+
+public class Message
+{
+    public Guid Id { get; set; }
+    public required string Content { get; set; }
+    public MessageType Type { get; set; } = MessageType.Text;
+    public string? AttachmentUrl { get; set; }
+    public string? AttachmentFileName { get; set; }
+    public DateTimeOffset SentAt { get; set; } = DateTimeOffset.UtcNow;
+
+    public Guid ChannelId { get; set; }
+    public Channel? Channel { get; set; }
+
+    public Guid SenderUserId { get; set; }
+    public required string SenderUsername { get; set; }
+}