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.Server/Services/FileStorageService.cs

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+namespace EchoHub.Server.Services;
+
+public class FileStorageService
+{
+    private readonly string _storagePath;
+
+    public FileStorageService(IConfiguration configuration)
+    {
+        _storagePath = configuration["Storage:Path"] ?? "./uploads";
+
+        if (!Directory.Exists(_storagePath))
+        {
+            Directory.CreateDirectory(_storagePath);
+        }
+    }
+
+    public async Task<(string fileId, string filePath)> SaveFileAsync(Stream stream, string fileName)
+    {
+        var fileId = Guid.NewGuid().ToString();
+        var extension = Path.GetExtension(fileName);
+        var storedFileName = $"{fileId}{extension}";
+        var filePath = Path.Combine(_storagePath, storedFileName);
+
+        using var fileStream = File.Create(filePath);
+        await stream.CopyToAsync(fileStream);
+
+        return (fileId, filePath);
+    }
+
+    public string? GetFilePath(string fileId)
+    {
+        var files = Directory.GetFiles(_storagePath, $"{fileId}.*");
+
+        return files.Length > 0 ? files[0] : null;
+    }
+
+    public void DeleteFile(string fileId)
+    {
+        var filePath = GetFilePath(fileId);
+
+        if (filePath is not null && File.Exists(filePath))
+        {
+            File.Delete(filePath);
+        }
+    }
+}

src/EchoHub.Server/Services/ImageToAsciiService.cs

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+using System.Text;
+using EchoHub.Core.Constants;
+using SixLabors.ImageSharp;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing;
+
+namespace EchoHub.Server.Services;
+
+public class ImageToAsciiService
+{
+    private static readonly char[] AsciiChars = " .:-=+*#%@".ToCharArray();
+
+    public string ConvertToAscii(Stream imageStream, int width = HubConstants.AsciiArtWidth, int height = HubConstants.AsciiArtHeight)
+    {
+        using var image = Image.Load<Rgba32>(imageStream);
+
+        image.Mutate(x => x.Resize(width, height));
+
+        var sb = new StringBuilder();
+
+        for (int y = 0; y < image.Height; y++)
+        {
+            for (int x = 0; x < image.Width; x++)
+            {
+                var pixel = image[x, y];
+                var brightness = 0.299 * pixel.R + 0.587 * pixel.G + 0.114 * pixel.B;
+
+                // Map brightness (0-255) to ASCII char index (inverted: dark pixels get dense chars)
+                var index = (int)((brightness / 255.0) * (AsciiChars.Length - 1));
+                sb.Append(AsciiChars[index]);
+            }
+
+            if (y < image.Height - 1)
+            {
+                sb.AppendLine();
+            }
+        }
+
+        return sb.ToString();
+    }
+}

src/EchoHub.Server/Services/PresenceTracker.cs

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+using System.Collections.Concurrent;
+
+namespace EchoHub.Server.Services;
+
+public class PresenceTracker
+{
+    private readonly ConcurrentDictionary<string, (Guid userId, string username)> _connections = new();
+    private readonly ConcurrentDictionary<string, HashSet<string>> _userConnections = new();
+    private readonly ConcurrentDictionary<string, HashSet<string>> _userChannels = new();
+
+    private readonly object _lock = new();
+
+    public void UserConnected(string connectionId, Guid userId, string username)
+    {
+        _connections[connectionId] = (userId, username);
+
+        lock (_lock)
+        {
+            if (!_userConnections.TryGetValue(username, out var connections))
+            {
+                connections = new HashSet<string>();
+                _userConnections[username] = connections;
+            }
+
+            connections.Add(connectionId);
+        }
+    }
+
+    public string? UserDisconnected(string connectionId)
+    {
+        if (!_connections.TryRemove(connectionId, out var userInfo))
+            return null;
+
+        var username = userInfo.username;
+
+        lock (_lock)
+        {
+            if (_userConnections.TryGetValue(username, out var connections))
+            {
+                connections.Remove(connectionId);
+
+                if (connections.Count == 0)
+                {
+                    _userConnections.TryRemove(username, out _);
+                    _userChannels.TryRemove(username, out _);
+                }
+            }
+        }
+
+        return username;
+    }
+
+    public void JoinChannel(string username, string channelName)
+    {
+        lock (_lock)
+        {
+            if (!_userChannels.TryGetValue(username, out var channels))
+            {
+                channels = new HashSet<string>();
+                _userChannels[username] = channels;
+            }
+
+            channels.Add(channelName);
+        }
+    }
+
+    public void LeaveChannel(string username, string channelName)
+    {
+        lock (_lock)
+        {
+            if (_userChannels.TryGetValue(username, out var channels))
+            {
+                channels.Remove(channelName);
+            }
+        }
+    }
+
+    public List<string> GetOnlineUsersInChannel(string channelName)
+    {
+        var users = new List<string>();
+
+        lock (_lock)
+        {
+            foreach (var (username, channels) in _userChannels)
+            {
+                if (channels.Contains(channelName))
+                {
+                    users.Add(username);
+                }
+            }
+        }
+
+        return users;
+    }
+
+    public List<string> GetChannelsForUser(string username)
+    {
+        lock (_lock)
+        {
+            if (_userChannels.TryGetValue(username, out var channels))
+            {
+                return channels.ToList();
+            }
+        }
+
+        return [];
+    }
+
+    public bool IsOnline(string username)
+    {
+        return _userConnections.TryGetValue(username, out var connections) && connections.Count > 0;
+    }
+}