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/Views/ConfigCheckpointsDialog.xaml.cs

@@ -0,0 +1,19 @@
+using System.Windows;
+using WebhookServer.Gui.ViewModels;
+
+namespace WebhookServer.Gui.Views;
+
+public partial class ConfigCheckpointsDialog : Window
+{
+    public ConfigCheckpointsDialog()
+    {
+        InitializeComponent();
+        Loaded += async (_, _) =>
+        {
+            if (DataContext is ConfigCheckpointsViewModel vm)
+                await vm.RefreshAsync();
+        };
+    }
+
+    private void OnClose(object sender, RoutedEventArgs e) => Close();
+}