feat: add true transparent background support via Terminal.Gui fork

Add Terminal.Gui fork as submodule with transparent color support
(Color.Transparent emitting CSI 39m/49m default bg/fg sequences).
Switch EchoHub.Client from NuGet to project reference.
Update Transparent theme to use real terminal transparency.

src/EchoHub.Client/EchoHub.Client.csproj

@@ -11,7 +11,8 @@
     <PackageReference Include="Serilog" Version="4.3.1" />
     <PackageReference Include="Serilog.Settings.Configuration" Version="10.0.0" />
     <PackageReference Include="Serilog.Sinks.File" Version="7.0.0" />
-    <PackageReference Include="Terminal.Gui" Version="2.0.0-develop.5027" />
+    <!-- Using local fork until transparent color support is merged upstream (gui-cs/Terminal.Gui#4234) -->
+    <ProjectReference Include="..\Terminal.Gui\Terminal.Gui\Terminal.Gui.csproj" />
   </ItemGroup>
 
   <ItemGroup>

src/EchoHub.Client/Themes/ThemeManager.cs

@@ -418,30 +418,30 @@ public static class ThemeManager
         Base = new ThemeColors
         {
             Foreground = "White",
-            Background = "Black",
+            Background = "Transparent",
             FocusForeground = "BrightCyan",
-            FocusBackground = "Black"
+            FocusBackground = "Transparent"
         },
         Menu = new ThemeColors
         {
             Foreground = "White",
-            Background = "Black",
+            Background = "Transparent",
             FocusForeground = "BrightCyan",
-            FocusBackground = "Black"
+            FocusBackground = "Transparent"
         },
         Dialog = new ThemeColors
         {
             Foreground = "White",
-            Background = "Black",
+            Background = "DarkGray",
             FocusForeground = "BrightCyan",
             FocusBackground = "Black"
         },
         Status = new ThemeColors
         {
             Foreground = "Gray",
-            Background = "Black",
+            Background = "Transparent",
             FocusForeground = "Gray",
-            FocusBackground = "Black"
+            FocusBackground = "Transparent"
         }
     };