Rename "Backups" menu item to "Config Checkpoints"

User-facing copy only; internal API names (Backups collection,
BackupEntry, list-backups op, etc.) stay the same to avoid churn
through the wire protocol and existing on-disk files. The new
phrasing makes the auto-snapshot-before-save model more discoverable.

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

src/WebhookServer.Gui/MainWindow.xaml

@@ -29,8 +29,9 @@
                 <Separator/>
                 <MenuItem Header="_Import config…" Command="{Binding ImportConfigCommand}"/>
                 <MenuItem Header="_Export config…" Command="{Binding ExportConfigCommand}"/>
-                <MenuItem Header="_Backups"
+                <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">