fix(gutter): enable mouse events for tooltip

Add transparent background to gutter margin Canvas so mouse events
are properly captured. WPF requires a Background to be set for
hit-testing to work.

Author: CalvinAllen

src/CodingWithCalvin.GitRanger/Editor/GutterMargin/BlameMargin.cs

@@ -44,6 +44,7 @@ public BlameMargin(IWpfTextView view, ITextDocumentFactoryService? textDocumentF
             var options = GeneralOptions.Instance;
             Width = options?.GutterWidth ?? 40;
             ClipToBounds = true;
+            Background = Brushes.Transparent; // Required for mouse events to work
 
             // Ensure services are initialized (in case package hasn't loaded yet)
             GitRangerPackage.EnsureServicesInitialized();