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

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+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;
+}

src/EchoHub.Core/Contracts/IEchoHubClient.cs

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+using EchoHub.Core.DTOs;
+
+namespace EchoHub.Core.Contracts;
+
+/// <summary>
+/// Methods the server can invoke on connected clients.
+/// </summary>
+public interface IEchoHubClient
+{
+    Task ReceiveMessage(MessageDto message);
+    Task UserJoined(string channelName, string username);
+    Task UserLeft(string channelName, string username);
+    Task ChannelUpdated(ChannelDto channel);
+    Task UserStatusChanged(UserPresenceDto presence);
+    Task Error(string message);
+}

src/EchoHub.Core/DTOs/AuthDtos.cs

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+namespace EchoHub.Core.DTOs;
+
+public record RegisterRequest(string Username, string Password, string? DisplayName = null);
+
+public record LoginRequest(string Username, string Password);
+
+public record LoginResponse(string Token, string Username, string? DisplayName, string? NicknameColor);

src/EchoHub.Core/DTOs/ChatDtos.cs

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+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);

src/EchoHub.Core/DTOs/ProfileDtos.cs

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+using EchoHub.Core.Models;
+
+namespace EchoHub.Core.DTOs;
+
+public record UserProfileDto(
+    Guid Id,
+    string Username,
+    string? DisplayName,
+    string? Bio,
+    string? NicknameColor,
+    string? AvatarAscii,
+    UserStatus Status,
+    string? StatusMessage,
+    DateTimeOffset CreatedAt,
+    DateTimeOffset LastSeenAt);
+
+public record UpdateProfileRequest(
+    string? DisplayName = null,
+    string? Bio = null,
+    string? NicknameColor = null);
+
+public record UpdateStatusRequest(
+    UserStatus Status,
+    string? StatusMessage = null);
+
+public record UserPresenceDto(
+    string Username,
+    string? DisplayName,
+    string? NicknameColor,
+    UserStatus Status,
+    string? StatusMessage);

src/EchoHub.Core/DTOs/ServerDtos.cs

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+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);

src/EchoHub.Core/EchoHub.Core.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

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
+}