feat(ui): add aggregated diff viewer for planning tasks

Implements Task 14: PlanningDiffView (Window), PlanningDiffViewModel,
ShowPlanningDiffModal callback wired in DetailsIslandView, and 5 xUnit tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/Planning/PlanningDiffView.axaml.cs

@@ -0,0 +1,27 @@
+using Avalonia.Controls;
+using Avalonia.Input;
+using CommunityToolkit.Mvvm.Input;
+using ClaudeDo.Ui.ViewModels.Planning;
+
+namespace ClaudeDo.Ui.Views.Planning;
+
+public partial class PlanningDiffView : Window
+{
+    public PlanningDiffView()
+    {
+        InitializeComponent();
+    }
+
+    protected override void OnDataContextChanged(EventArgs e)
+    {
+        base.OnDataContextChanged(e);
+        if (DataContext is PlanningDiffViewModel vm)
+            vm.CloseAction = Close;
+    }
+
+    private void TitleBar_PointerPressed(object? sender, PointerPressedEventArgs e)
+    {
+        if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
+            BeginMoveDrag(e);
+    }
+}