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/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);