Extract a per-user background host so indexing can later run outside the window.

Keep the GUI as the index writer for now; mutex, named pipe, and opt-in logon autostart prepare Explorer.Host.exe without two SQLite writers.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/Explorer.App/SettingsWindow.xaml.cs

@@ -1,5 +1,6 @@
 using System.Windows;
 using Explorer.Application;
+using Explorer.Hosting;
 using Explorer.Presentation.ViewModels;
 
 namespace Explorer.App;
@@ -20,6 +21,8 @@ public partial class SettingsWindow : Window
         GroupNetwork.IsChecked = prefs.GroupNetworkPlaces;
         GroupCloud.IsChecked = prefs.GroupCloudPlaces;
         IndexArchives.IsChecked = prefs.IndexArchiveContents;
+        AutoIndexRemovable.IsChecked = prefs.AutoIndexRemovable;
+        BackgroundHostAtLogon.IsChecked = prefs.BackgroundHostAtLogon;
         ShowHidden.IsChecked = prefs.ShowHiddenFiles;
         ShowProtected.IsChecked = prefs.ShowProtectedSystemLocations;
         AutoClearQueue.IsChecked = prefs.AutoClearQueueWhenDone;
@@ -45,6 +48,8 @@ public partial class SettingsWindow : Window
             GroupNetworkPlaces = GroupNetwork.IsChecked == true,
             GroupCloudPlaces = GroupCloud.IsChecked == true,
             IndexArchiveContents = IndexArchives.IsChecked == true,
+            AutoIndexRemovable = AutoIndexRemovable.IsChecked == true,
+            BackgroundHostAtLogon = BackgroundHostAtLogon.IsChecked == true,
             ShowHiddenFiles = ShowHidden.IsChecked == true,
             ShowProtectedSystemLocations = ShowProtected.IsChecked == true,
             AutoClearQueueWhenDone = AutoClearQueue.IsChecked == true,
@@ -52,10 +57,41 @@ public partial class SettingsWindow : Window
             GitPath = string.IsNullOrWhiteSpace(GitPath.Text) ? null : GitPath.Text.Trim()
         };
         await _vm.ApplyPreferencesAsync(prefs).ConfigureAwait(true);
+        ApplyBackgroundHostAutostart(prefs.BackgroundHostAtLogon);
         DialogResult = true;
         Close();
     }
 
+    private void ApplyBackgroundHostAutostart(bool enabled)
+    {
+        if (enabled)
+        {
+            var exe = HostLogonAutostart.FindHostExecutable();
+            if (exe is null)
+            {
+                MessageBox.Show(
+                    this,
+                    "Explorer.Host.exe was not found next to Explorer Workbench, so the sign-in task was not registered.",
+                    "Background host",
+                    MessageBoxButton.OK,
+                    MessageBoxImage.Warning);
+                return;
+            }
+
+            if (!HostLogonAutostart.TryRegister(exe, out var error))
+            {
+                MessageBox.Show(this, error, "Background host", MessageBoxButton.OK, MessageBoxImage.Warning);
+            }
+
+            return;
+        }
+
+        if (!HostLogonAutostart.TryUnregister(out var unregisterError))
+        {
+            MessageBox.Show(this, unregisterError, "Background host", MessageBoxButton.OK, MessageBoxImage.Warning);
+        }
+    }
+
     private void OnBrowseSevenZip(object sender, RoutedEventArgs e)
     {
         var dlg = new Microsoft.Win32.OpenFileDialog