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/UI/Chat/RenderHelpers.cs

@@ -0,0 +1,27 @@
+using Terminal.Gui.Drawing;
+using Terminal.Gui.Text;
+using Terminal.Gui.Views;
+
+namespace EchoHub.Client.UI.Chat;
+
+/// <summary>
+/// Shared rendering helpers for IListDataSource implementations.
+/// </summary>
+static class RenderHelpers
+{
+    /// <summary>
+    /// Write text grapheme-by-grapheme to a ListView, respecting a width limit.
+    /// Returns the updated drawn-columns count.
+    /// </summary>
+    public static int WriteText(ListView lv, string text, int drawn, int maxWidth)
+    {
+        foreach (var grapheme in GraphemeHelper.GetGraphemes(text))
+        {
+            var cols = Math.Max(grapheme.GetColumns(), 1);
+            if (drawn + cols > maxWidth) break;
+            lv.AddStr(grapheme);
+            drawn += cols;
+        }
+        return drawn;
+    }
+}