refactor(ui): dedupe path-open helper across five view models

ListsIslandViewModel.OpenInExplorer, MergeSectionViewModel.OpenWorktree,
WorktreesOverviewModalViewModel.OpenInExplorer, AboutModalViewModel.OpenPath and
TasksIslandViewModel.OpenTaskWorktree each reimplemented "open this path in the
shell" with their own existence check, launch mechanism and error handling.
Replace all five with the new ShellOpen.Path helper and route failures through
the existing ErrorReported -> footer-strip convention instead of bare catch {}.

src/ClaudeDo.Ui/ViewModels/IslandsShellViewModel.cs

@@ -544,6 +544,7 @@ public sealed partial class IslandsShellViewModel : ViewModelBase, IDisposable
     private async Task OpenAbout()
     {
         var vm = new AboutModalViewModel();
+        vm.ErrorReported += FlashFooterError;
         if (Dialogs is not null) await Dialogs.ShowAboutAsync(vm);
     }
 
@@ -599,6 +600,7 @@ public sealed partial class IslandsShellViewModel : ViewModelBase, IDisposable
         try
         {
             var vm = _worktreesOverviewVmFactory();
+            vm.ErrorReported += FlashFooterError;
             vm.Configure(null, null);
             await vm.LoadAsync();
             await Dialogs.ShowWorktreesOverviewAsync(vm);