feat(ui): add conflict resolution dialog for planning merge-all

Opens a modal when PlanningMergeConflict fires, listing conflicted files
with options to open in VS Code, continue, or abort the merge.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/MainWindow.axaml.cs

@@ -1,6 +1,7 @@
 using Avalonia.Controls;
 using Avalonia.Input;
 using ClaudeDo.Ui.ViewModels;
+using ClaudeDo.Ui.Views.Planning;
 
 namespace ClaudeDo.Ui.Views;
 
@@ -10,6 +11,19 @@ public partial class MainWindow : Window
     {
         InitializeComponent();
         KeyDown += OnWindowKeyDown;
+        DataContextChanged += OnDataContextChanged;
+    }
+
+    private void OnDataContextChanged(object? sender, EventArgs e)
+    {
+        if (DataContext is IslandsShellViewModel vm)
+        {
+            vm.ShowConflictDialog = async (conflictVm) =>
+            {
+                var modal = new ConflictResolutionView { DataContext = conflictVm };
+                await modal.ShowDialog(this);
+            };
+        }
     }
 
     private void OnWindowKeyDown(object? sender, KeyEventArgs e)