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.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "src/Terminal.Gui"]
+	path = src/Terminal.Gui
+	url = https://github.com/HueByte/Terminal.Gui.git

docs/changelog/v0.2.5.md

@@ -29,6 +29,14 @@
 - Migrates legacy single-object `EmbedJson` to array format
 - Promotes usernames listed in `Server:Admins` config to Admin role
 
+### True Transparent Background
+
+- Transparent theme now uses the terminal's native background instead of solid black
+- Powered by `Color.Transparent` (alpha=0) which emits ANSI `CSI 49m` (default background) instead of explicit RGB
+- Terminal transparency, acrylic, wallpaper effects now show through the TUI
+- Dialogs retain solid `DarkGray` background for readability
+- Uses local Terminal.Gui fork (submodule) with transparent color support pending upstream merge ([gui-cs/Terminal.Gui#4234](https://github.com/gui-cs/Terminal.Gui/pull/4234))
+
 ## Fixes
 
 ### IRC Gateway

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"
         }
     };