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/EchoHub.Client.csproj

@@ -20,7 +20,6 @@
     <Content Include="appsettings.json" Condition="Exists('appsettings.json')">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
-    <Content Include="hue_icon.ico" />
     <Content Include="Assets\**">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
@@ -35,7 +34,7 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
     <PackageIcon></PackageIcon>
-    <ApplicationIcon>hue_icon.ico</ApplicationIcon>
+    <ApplicationIcon>Assets\hue_icon.ico</ApplicationIcon>
   </PropertyGroup>
 
 </Project>