feat(ui): confirm main-window close when Mission Control sessions are open

Closing the main window used to silently kill every open Mission Control
ConPTY session (App's ShutdownMode.OnMainWindowClose tears the process down
without warning). Intercept Window.Closing, and when at least one pane is
open, ask via the existing ConfirmAsync pattern before proceeding; cancelling
leaves the window and sessions alive, confirming force-closes as before.

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

@@ -1,9 +1,12 @@
 using System.Linq;
+using System.Threading.Tasks;
 using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.Media;
+using ClaudeDo.Ui.Localization;
+using ClaudeDo.Ui.Services;
 using ClaudeDo.Ui.ViewModels;
 using ClaudeDo.Ui.ViewModels.Islands;
 using ClaudeDo.Ui.ViewModels.Modals;
@@ -13,6 +16,8 @@ namespace ClaudeDo.Ui.Views;
 
 public partial class MainWindow : Window
 {
+    private bool _forceClose;
+
     public MainWindow()
     {
         InitializeComponent();
@@ -21,6 +26,29 @@ public partial class MainWindow : Window
         UpdateMaxIcon();
     }
 
+    // Closing the main window kills every open Mission Control ConPTY session without
+    // warning (App.axaml.cs: ShutdownMode.OnMainWindowClose). Ask first — never silently.
+    protected override void OnClosing(WindowClosingEventArgs e)
+    {
+        if (!_forceClose
+            && DataContext is IslandsShellViewModel { Dialogs: { } dialogs } vm
+            && IslandsShellViewModel.RequiresCloseConfirmation(vm.OpenMissionControlSessionCount))
+        {
+            e.Cancel = true;
+            _ = ConfirmCloseWithOpenSessionsAsync(dialogs, vm.OpenMissionControlSessionCount);
+            return;
+        }
+        base.OnClosing(e);
+    }
+
+    private async Task ConfirmCloseWithOpenSessionsAsync(IDialogService dialogs, int openSessionCount)
+    {
+        var confirmed = await dialogs.ConfirmAsync(Loc.T("vm.mainWindow.closeConfirm.message", openSessionCount));
+        if (!confirmed) return;
+        _forceClose = true;
+        Close();
+    }
+
     protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
     {
         base.OnPropertyChanged(change);