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/AsyncRunner.cs

@@ -0,0 +1,32 @@
+using Serilog;
+using Terminal.Gui.App;
+
+namespace EchoHub.Client.Services;
+
+/// <summary>
+/// Eliminates repeated Task.Run/try/catch/app.Invoke(ShowError) boilerplate.
+/// Runs async work on a background thread and routes exceptions to the UI.
+/// </summary>
+public static class AsyncRunner
+{
+    public static void Run(
+        IApplication app,
+        Func<Task> work,
+        Action<string> showError,
+        string errorPrefix,
+        string? logContext = null)
+    {
+        Task.Run(async () =>
+        {
+            try
+            {
+                await work();
+            }
+            catch (Exception ex)
+            {
+                Log.Error(ex, "{Context} failed", logContext ?? errorPrefix);
+                app.Invoke(() => showError($"{errorPrefix}: {ex.Message}"));
+            }
+        });
+    }
+}