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/Modals/WorktreesOverviewModalViewModel.cs

@@ -1,5 +1,4 @@
 using System.Collections.ObjectModel;
-using System.Diagnostics;
 using Avalonia;
 using Avalonia.Controls.ApplicationLifetimes;
 using Avalonia.Input.Platform;
@@ -94,6 +93,10 @@ public sealed partial class WorktreesOverviewModalViewModel : ViewModelBase
     public ObservableCollection<string> MergeTargets { get; } = new();
     public ObservableCollection<WorktreeOverviewRowViewModel> ConflictRows { get; } = new();
 
+    // Mirrors UsageMonitorModalViewModel.ErrorReported — surfaces modal-owned failures in the
+    // footer strip once the caller wires it (the modal's own window sits in front of the strip).
+    public event Action<string>? ErrorReported;
+
     public Action? CloseAction { get; set; }
     public Action<DiffViewerViewModel>? ShowDiffAction { get; set; }
     public Action<string, string>? JumpToTaskAction { get; set; }
@@ -262,9 +265,9 @@ public sealed partial class WorktreesOverviewModalViewModel : ViewModelBase
     [RelayCommand]
     private void OpenInExplorer(WorktreeOverviewRowViewModel? row)
     {
-        if (row is null || !row.PathExistsOnDisk) return;
-        try { Process.Start(new ProcessStartInfo { FileName = row.Path, UseShellExecute = true }); }
-        catch { }
+        var (ok, error) = ShellOpen.Path(row?.Path);
+        if (!ok && error is not null)
+            ErrorReported?.Invoke(Loc.T("vm.worktreesOverview.openInExplorerFailed", error));
     }
 
     [RelayCommand]