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/Islands/ListsIslandViewModel.cs

@@ -80,6 +80,7 @@ public sealed partial class ListsIslandViewModel : ViewModelBase, IDisposable
         {
             var rawId = row.Id.StartsWith("user:", StringComparison.Ordinal) ? row.Id["user:".Length..] : row.Id;
             var vm = _services.GetRequiredService<WorktreesOverviewModalViewModel>();
+            vm.ErrorReported += msg => ErrorReported?.Invoke(msg);
             vm.Configure(rawId, row.Name);
             await vm.LoadAsync();
             await Dialogs.ShowWorktreesOverviewAsync(vm);
@@ -107,17 +108,9 @@ public sealed partial class ListsIslandViewModel : ViewModelBase, IDisposable
     [RelayCommand]
     private void OpenInExplorer(ListNavItemViewModel? row)
     {
-        var dir = row?.WorkingDir;
-        if (string.IsNullOrWhiteSpace(dir) || !System.IO.Directory.Exists(dir)) return;
-        try
-        {
-            System.Diagnostics.Process.Start(new System.Diagnostics.ProcessStartInfo
-            {
-                FileName = dir,
-                UseShellExecute = true,
-            });
-        }
-        catch { /* best-effort */ }
+        var (ok, error) = ShellOpen.Path(row?.WorkingDir);
+        if (!ok && error is not null)
+            ErrorReported?.Invoke(Loc.T("vm.lists.openInExplorerFailed", error));
     }
 
     [RelayCommand]