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

@@ -29,25 +29,7 @@
                 <Separator/>
                 <MenuItem Header="_Import config…" Command="{Binding ImportConfigCommand}"/>
                 <MenuItem Header="_Export config…" Command="{Binding ExportConfigCommand}"/>
-                <MenuItem Header="Config _Checkpoints"
-                          ItemsSource="{Binding Backups}"
-                          ToolTip="Snapshots taken automatically before each save. Click one to restore."
-                          SubmenuOpened="OnBackupsSubmenuOpened">
-                    <MenuItem.ItemContainerStyle>
-                        <Style TargetType="MenuItem">
-                            <Setter Property="Header">
-                                <Setter.Value>
-                                    <MultiBinding StringFormat="{}{0:yyyy-MM-dd HH:mm:ss}  ({1:n0} bytes)">
-                                        <Binding Path="SavedAt"/>
-                                        <Binding Path="SizeBytes"/>
-                                    </MultiBinding>
-                                </Setter.Value>
-                            </Setter>
-                            <Setter Property="Command" Value="{Binding DataContext.RestoreBackupCommand, RelativeSource={RelativeSource AncestorType=Window}}"/>
-                            <Setter Property="CommandParameter" Value="{Binding}"/>
-                        </Style>
-                    </MenuItem.ItemContainerStyle>
-                </MenuItem>
+                <MenuItem Header="Config _Checkpoints…" Command="{Binding ShowConfigCheckpointsCommand}"/>
                 <Separator/>
                 <MenuItem Header="E_xit" Command="{Binding ExitCommand}"/>
             </MenuItem>