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.Server/EchoHub.Server.csproj

@@ -1,9 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk.Web">
 
-  <ItemGroup>
-    <Content Include="hue_icon.ico" />
-  </ItemGroup>
-
   <ItemGroup>
     <ProjectReference Include="..\EchoHub.Core\EchoHub.Core.csproj" />
     <ProjectReference Include="..\EchoHub.Server.Irc\EchoHub.Server.Irc.csproj" />
@@ -27,7 +23,7 @@
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
     <PackageIcon></PackageIcon>
-    <ApplicationIcon>hue_icon.ico</ApplicationIcon>
+    <ApplicationIcon>..\EchoHub.Client\Assets\hue_icon.ico</ApplicationIcon>
   </PropertyGroup>
 
 </Project>