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/Channel.cs

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+namespace EchoHub.Core.Models;
+
+public class Channel
+{
+    public Guid Id { get; set; }
+    public required string Name { get; set; }
+    public string? Topic { get; set; }
+    public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
+    public Guid CreatedByUserId { get; set; }
+
+    public List<Message> Messages { get; set; } = [];
+}

src/EchoHub.Core/Models/Message.cs

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+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; }
+}

src/EchoHub.Core/Models/MessageType.cs

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+namespace EchoHub.Core.Models;
+
+public enum MessageType
+{
+    Text,
+    Image,
+    File
+}

src/EchoHub.Core/Models/ServerInfo.cs

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+namespace EchoHub.Core.Models;
+
+public class ServerInfo
+{
+    public Guid Id { get; set; }
+    public required string Name { get; set; }
+    public string? Description { get; set; }
+    public required string Url { get; set; }
+    public int OnlineUsers { get; set; }
+    public int TotalChannels { get; set; }
+    public DateTimeOffset RegisteredAt { get; set; } = DateTimeOffset.UtcNow;
+    public DateTimeOffset LastPingAt { get; set; } = DateTimeOffset.UtcNow;
+    public bool IsOnline { get; set; } = true;
+}

src/EchoHub.Core/Models/User.cs

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+namespace EchoHub.Core.Models;
+
+public class User
+{
+    public Guid Id { get; set; }
+    public required string Username { get; set; }
+    public required string PasswordHash { get; set; }
+    public string? DisplayName { get; set; }
+    public string? Bio { get; set; }
+    public string? NicknameColor { get; set; }
+    public string? AvatarAscii { get; set; }
+    public UserStatus Status { get; set; } = UserStatus.Online;
+    public string? StatusMessage { get; set; }
+    public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
+    public DateTimeOffset LastSeenAt { get; set; } = DateTimeOffset.UtcNow;
+}

src/EchoHub.Core/Models/UserStatus.cs

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+namespace EchoHub.Core.Models;
+
+public enum UserStatus
+{
+    Online,
+    Away,
+    DoNotDisturb,
+    Invisible
+}