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/DTOs/ServerDtos.cs

@@ -0,0 +1,15 @@
+namespace EchoHub.Core.DTOs;
+
+public record ServerInfoDto(
+    Guid Id,
+    string Name,
+    string? Description,
+    string Url,
+    int OnlineUsers,
+    int TotalChannels,
+    bool IsOnline,
+    DateTimeOffset LastPingAt);
+
+public record RegisterServerRequest(string Name, string Url, string? Description = null);
+
+public record ServerStatusDto(string Name, string? Description, int OnlineUsers, int TotalChannels);