fix(ui): open diff viewer and merge window as independent top-level windows

Owned windows are pinned above their owner by Windows, so the main window
could never come to the front. Show() without an owner and centre manually.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/Modals/DiffViewerView.axaml

@@ -11,7 +11,6 @@
         WindowDecorations="BorderOnly"
         ExtendClientAreaToDecorationsHint="True"
         ExtendClientAreaTitleBarHeightHint="-1"
-        WindowStartupLocation="CenterOwner"
         Background="{DynamicResource SurfaceBrush}">
 
   <Window.KeyBindings>

src/ClaudeDo.Ui/Views/Modals/MergeModalView.axaml

@@ -11,7 +11,6 @@
         WindowDecorations="BorderOnly"
         ExtendClientAreaToDecorationsHint="True"
         ExtendClientAreaTitleBarHeightHint="-1"
-        WindowStartupLocation="CenterOwner"
         Background="{DynamicResource SurfaceBrush}">
 
   <Window.KeyBindings>

src/ClaudeDo.Ui/Views/Modals/MergeModalView.axaml.cs

@@ -41,7 +41,9 @@ public partial class MergeModalView : Window
             OpenWindows.Remove(vm.TaskId);
             window._closed.TrySetResult();
         };
-        window.Show(owner);
+        // Independent top-level (no owner): an owned window is pinned above its owner by the OS.
+        window.CenterOn(owner);
+        window.Show();
         return window._closed.Task;
     }
 }