Phase 4: backups + import/export config

ConfigStore.SaveAsync now snapshots the previous config to
%ProgramData%\WebhookServer\backups\config-<timestamp>.json before
overwriting, retaining the last 30. Failures are silent so a
backup-write hiccup never blocks an actual save.

Three new admin pipe ops:
- list-backups: returns newest 50 entries with timestamps and sizes
- restore-backup: takes a fileName, refuses path-traversal chars,
  loads the named backup over the live config (which itself triggers
  a fresh backup of the current state via the SaveAsync hook)
- import-config: replaces the current config with a GUI-supplied
  ServerConfig, merging encrypted secrets where the GUI didn't supply
  new plaintext

GUI File menu items are wired:
- Import config: file picker -> ImportConfigAsync
- Export config: SaveFileDialog writes the current config as JSON
- Backups: dynamic submenu auto-refreshed when opened, listing
  backups with timestamp + size; click to confirm-and-restore

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

src/WebhookServer.Gui/MainWindow.xaml.cs

@@ -52,4 +52,10 @@ public partial class MainWindow : Window
         if (DataContext is MainViewModel vm && vm.EditEndpointCommand.CanExecute(null))
             vm.EditEndpointCommand.Execute(null);
     }
+
+    private async void OnBackupsSubmenuOpened(object sender, RoutedEventArgs e)
+    {
+        if (DataContext is MainViewModel vm)
+            await vm.RefreshBackupsCommand.ExecuteAsync(null);
+    }
 }