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/Constants/HubConstants.cs

@@ -0,0 +1,13 @@
+namespace EchoHub.Core.Constants;
+
+public static class HubConstants
+{
+    public const string ChatHubPath = "/hubs/chat";
+    public const string DefaultChannel = "general";
+    public const int DefaultHistoryCount = 50;
+    public const int MaxMessageLength = 2000;
+    public const int MaxFileSizeBytes = 10 * 1024 * 1024; // 10 MB
+    public const int MaxAvatarSizeBytes = 2 * 1024 * 1024; // 2 MB
+    public const int AsciiArtWidth = 80;
+    public const int AsciiArtHeight = 40;
+}