Add hex color parsing helper and implement custom list sources for channels and users

- Introduced HexColorHelper for parsing hex color strings to Terminal.Gui Attributes and Colors.
- Created ChannelListSource for managing and rendering a list of channels with unread counts and active indicators.
- Developed UserListSource for displaying online users with per-user nickname colors.
- Refactored MainWindow to utilize ChatMessageManager for handling messages and channel state.
- Updated project references and removed unused content from the server project.
- Adjusted tests to reflect changes in namespaces and structure.

src/EchoHub.Client/Services/AvatarHelper.cs

@@ -0,0 +1,44 @@
+using Serilog;
+
+namespace EchoHub.Client.Services;
+
+/// <summary>
+/// Shared avatar upload logic — resolves a file path or URL to a stream
+/// and uploads it via ApiClient.
+/// </summary>
+internal static class AvatarHelper
+{
+    /// <summary>
+    /// Upload an avatar from a local file path or HTTP(S) URL.
+    /// Returns the ASCII art response from the server.
+    /// </summary>
+    public static async Task<string?> UploadAsync(ApiClient apiClient, string target)
+    {
+        Stream stream;
+        string fileName;
+
+        if (Uri.TryCreate(target, UriKind.Absolute, out var uri)
+            && (uri.Scheme == "http" || uri.Scheme == "https"))
+        {
+            using var http = new HttpClient();
+            var bytes = await http.GetByteArrayAsync(uri);
+            stream = new MemoryStream(bytes);
+            fileName = Path.GetFileName(uri.LocalPath);
+            if (string.IsNullOrWhiteSpace(fileName) || !fileName.Contains('.'))
+                fileName = "avatar.png";
+        }
+        else
+        {
+            if (!File.Exists(target))
+                throw new FileNotFoundException($"File not found: {target}");
+
+            stream = File.OpenRead(target);
+            fileName = Path.GetFileName(target);
+        }
+
+        await using (stream)
+        {
+            return await apiClient.UploadAvatarAsync(stream, fileName);
+        }
+    }
+}