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/ChatDtos.cs

@@ -0,0 +1,31 @@
+using EchoHub.Core.Models;
+
+namespace EchoHub.Core.DTOs;
+
+public record MessageDto(
+    Guid Id,
+    string Content,
+    string SenderUsername,
+    string? SenderNicknameColor,
+    string ChannelName,
+    MessageType Type,
+    string? AttachmentUrl,
+    string? AttachmentFileName,
+    DateTimeOffset SentAt);
+
+public record ChannelDto(
+    Guid Id,
+    string Name,
+    string? Topic,
+    int MessageCount,
+    DateTimeOffset CreatedAt);
+
+public record UserDto(
+    Guid Id,
+    string Username,
+    string? DisplayName,
+    string? NicknameColor,
+    UserStatus Status,
+    DateTimeOffset LastSeenAt);
+
+public record SendMessageRequest(string ChannelName, string Content);