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/ChatSegment.cs

@@ -0,0 +1,8 @@
+using Attribute = Terminal.Gui.Drawing.Attribute;
+
+namespace EchoHub.Client.UI.Chat;
+
+/// <summary>
+/// A colored text segment within a chat line.
+/// </summary>
+public record ChatSegment(string Text, Attribute? Color);