feat(merge): unify planning conflicts onto the resolver + 3-pane VM foundation

Route planning unit-merge conflicts through ConflictResolverViewModel
(OpenForPlanningAsync) and delete the old ConflictResolutionViewModel dialog.
Add active-file 3-pane reconstruction (MergeFile OursText/TheirsText/ResultText,
ActiveFile, SelectFileCommand, active-file readout) as the VM foundation for the
Rider-style editor. Seam preserved; Ui.Tests 128/128.

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

@@ -42,11 +42,6 @@ public partial class MainWindow : Window
     {
         if (DataContext is IslandsShellViewModel vm)
         {
-            vm.ShowConflictDialog = async (conflictVm) =>
-            {
-                var modal = new ConflictResolutionView { DataContext = conflictVm };
-                await modal.ShowDialog(this);
-            };
             vm.ShowAboutModal = async (aboutVm) =>
             {
                 var dlg = new AboutModalView { DataContext = aboutVm };