Config Checkpoints dialog + daily auto-checkpoint; drop installer GUI launch

Three fixes:

1. Config Checkpoints submenu replaced with a proper dialog. Lists
   checkpoints with timestamp/size/filename, has a "Take Checkpoint
   Now" button, and a "Roll Back" button that becomes enabled when a
   row is selected. The previous click-a-menu-entry-immediate-restore
   flow was too easy to fire by accident.

2. New CheckpointScheduler BackgroundService creates a checkpoint at
   midnight every day. Combined with the existing auto-on-save
   snapshots, this guarantees a daily rollback point even if the
   config wasn't edited that day. A new "create-checkpoint" admin op
   plus AdminPipeServer.CreateCheckpoint helper does the actual file
   copy; both manual (via the dialog) and the scheduler use it.

3. Installer: drop the post-install "Launch Webhook Server" wizard
   step. It tried to launch the GUI un-elevated, which fails because
   the GUI's manifest is requireAdministrator. The Start Menu shortcut
   handles elevation correctly, so the user can launch from there.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/WebhookServer.Gui/MainWindow.xaml.cs

@@ -53,9 +53,4 @@ public partial class MainWindow : Window
             vm.EditEndpointCommand.Execute(null);
     }
 
-    private async void OnBackupsSubmenuOpened(object sender, RoutedEventArgs e)
-    {
-        if (DataContext is MainViewModel vm)
-            await vm.RefreshBackupsCommand.ExecuteAsync(null);
-    }
 }