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

@@ -27,9 +27,26 @@
             <MenuItem Header="_File">
                 <MenuItem Header="_New endpoint…" Command="{Binding AddEndpointCommand}" InputGestureText="Ctrl+N"/>
                 <Separator/>
-                <MenuItem Header="_Import config…" IsEnabled="False" ToolTip="Coming soon"/>
-                <MenuItem Header="_Export config…" IsEnabled="False" ToolTip="Coming soon"/>
-                <MenuItem Header="_Backups" IsEnabled="False" ToolTip="Coming soon"/>
+                <MenuItem Header="_Import config…" Command="{Binding ImportConfigCommand}"/>
+                <MenuItem Header="_Export config…" Command="{Binding ExportConfigCommand}"/>
+                <MenuItem Header="_Backups"
+                          ItemsSource="{Binding Backups}"
+                          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>
                 <Separator/>
                 <MenuItem Header="E_xit" Command="{Binding ExitCommand}"/>
             </MenuItem>