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.Client/Services/ApiClient.cs

@@ -0,0 +1,172 @@
+using System.Net.Http.Headers;
+using System.Net.Http.Json;
+using System.Text.Json;
+using EchoHub.Core.DTOs;
+
+namespace EchoHub.Client.Services;
+
+public sealed class ApiClient : IDisposable
+{
+    private readonly HttpClient _http;
+    private string? _token;
+
+    public string? Token => _token;
+    public string BaseUrl { get; }
+
+    public ApiClient(string baseUrl)
+    {
+        BaseUrl = baseUrl.TrimEnd('/');
+        _http = new HttpClient
+        {
+            BaseAddress = new Uri(BaseUrl)
+        };
+    }
+
+    public async Task<LoginResponse> RegisterAsync(string username, string password, string? displayName = null)
+    {
+        var request = new RegisterRequest(username, password, displayName);
+        var response = await _http.PostAsJsonAsync("/api/auth/register", request);
+        await EnsureSuccessAsync(response);
+
+        var result = await response.Content.ReadFromJsonAsync<LoginResponse>()
+            ?? throw new InvalidOperationException("Registration returned empty response.");
+
+        SetToken(result.Token);
+        return result;
+    }
+
+    public async Task<LoginResponse> LoginAsync(string username, string password)
+    {
+        var request = new LoginRequest(username, password);
+        var response = await _http.PostAsJsonAsync("/api/auth/login", request);
+        await EnsureSuccessAsync(response);
+
+        var result = await response.Content.ReadFromJsonAsync<LoginResponse>()
+            ?? throw new InvalidOperationException("Login returned empty response.");
+
+        SetToken(result.Token);
+        return result;
+    }
+
+    public async Task<List<ChannelDto>> GetChannelsAsync()
+    {
+        EnsureAuthenticated();
+        var channels = await _http.GetFromJsonAsync<List<ChannelDto>>("/api/channels");
+        return channels ?? [];
+    }
+
+    public async Task<ServerStatusDto?> GetServerInfoAsync()
+    {
+        var info = await _http.GetFromJsonAsync<ServerStatusDto>("/api/server/info");
+        return info;
+    }
+
+    public async Task<UserProfileDto?> GetUserProfileAsync(string username)
+    {
+        EnsureAuthenticated();
+        var profile = await _http.GetFromJsonAsync<UserProfileDto>($"/api/users/{Uri.EscapeDataString(username)}/profile");
+        return profile;
+    }
+
+    public async Task<UserProfileDto?> UpdateProfileAsync(UpdateProfileRequest request)
+    {
+        EnsureAuthenticated();
+        var response = await _http.PutAsJsonAsync("/api/users/profile", request);
+        response.EnsureSuccessStatusCode();
+        return await response.Content.ReadFromJsonAsync<UserProfileDto>();
+    }
+
+    public async Task<string?> UploadAvatarAsync(Stream imageStream, string fileName)
+    {
+        EnsureAuthenticated();
+        using var content = new MultipartFormDataContent();
+        using var streamContent = new StreamContent(imageStream);
+        streamContent.Headers.ContentType = new MediaTypeHeaderValue(GetContentType(fileName));
+        content.Add(streamContent, "file", fileName);
+
+        var response = await _http.PostAsync("/api/users/avatar", content);
+        response.EnsureSuccessStatusCode();
+        var result = await response.Content.ReadFromJsonAsync<AvatarUploadResponse>();
+        return result?.AsciiArt;
+    }
+
+    public async Task<MessageDto?> UploadFileAsync(string channelName, Stream fileStream, string fileName)
+    {
+        EnsureAuthenticated();
+        using var content = new MultipartFormDataContent();
+        using var streamContent = new StreamContent(fileStream);
+        streamContent.Headers.ContentType = new MediaTypeHeaderValue(GetContentType(fileName));
+        content.Add(streamContent, "file", fileName);
+
+        var response = await _http.PostAsync($"/api/channels/{Uri.EscapeDataString(channelName)}/upload", content);
+        response.EnsureSuccessStatusCode();
+        return await response.Content.ReadFromJsonAsync<MessageDto>();
+    }
+
+    private void SetToken(string token)
+    {
+        _token = token;
+        _http.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
+    }
+
+    private static async Task EnsureSuccessAsync(HttpResponseMessage response)
+    {
+        if (response.IsSuccessStatusCode)
+            return;
+
+        // Try to extract a meaningful error message from the response body
+        var errorMessage = $"{(int)response.StatusCode} {response.ReasonPhrase}";
+        try
+        {
+            var body = await response.Content.ReadAsStringAsync();
+            if (!string.IsNullOrWhiteSpace(body))
+            {
+                // Try to parse {"error": "..."} format
+                using var doc = JsonDocument.Parse(body);
+                if (doc.RootElement.TryGetProperty("error", out var errorProp) ||
+                    doc.RootElement.TryGetProperty("Error", out errorProp))
+                {
+                    errorMessage = errorProp.GetString() ?? errorMessage;
+                }
+                else
+                {
+                    errorMessage = body;
+                }
+            }
+        }
+        catch
+        {
+            // If we can't parse the body, use the status code message
+        }
+
+        throw new HttpRequestException(errorMessage);
+    }
+
+    private void EnsureAuthenticated()
+    {
+        if (string.IsNullOrEmpty(_token))
+            throw new InvalidOperationException("Not authenticated. Call LoginAsync or RegisterAsync first.");
+    }
+
+    private static string GetContentType(string fileName)
+    {
+        var ext = Path.GetExtension(fileName).ToLowerInvariant();
+        return ext switch
+        {
+            ".jpg" or ".jpeg" => "image/jpeg",
+            ".png" => "image/png",
+            ".gif" => "image/gif",
+            ".webp" => "image/webp",
+            ".txt" => "text/plain",
+            ".pdf" => "application/pdf",
+            _ => "application/octet-stream",
+        };
+    }
+
+    public void Dispose()
+    {
+        _http.Dispose();
+    }
+}
+
+internal record AvatarUploadResponse(string AsciiArt);