feat(settings): make slide-out alerts persist until dismissed

Add a persisted global App Settings switch. Off keeps the existing per-severity
lifetimes (errors remain sticky); on makes every alert non-expiring and closes
it when its body is clicked. The NotificationService supplies the current
setting to each toast request.

StickyNotificationIntegrationTests covers persistence and both request modes.
618 tests green, clean build with 0 warnings.

Reference: Notification.Wpf exposes a CloseOnClick request option:
https://github.com/G-Stas/Notification.Wpf

Controls/SettingsDrawer.xaml

@@ -38,6 +38,11 @@
                 <ComboBoxItem Content="Bottom left" Tag="{x:Static notification:NotificationPosition.BottomLeft}"/>
                 <ComboBoxItem Content="Bottom right" Tag="{x:Static notification:NotificationPosition.BottomRight}"/>
             </ComboBox>
+            <CheckBox Content="Keep alerts until dismissed" Style="{StaticResource PropertyPillCheck}"
+                      Margin="12,2,0,2" IsChecked="{Binding StickyAlertsUntilDismissed, Mode=TwoWay}"
+                      ToolTip="When on, every slide-out alert stays until you click it or close it."/>
+            <TextBlock Text="Off: informational alerts use their normal timeouts; errors still stay until closed."
+                       Style="{StaticResource PropertyHint}" Margin="10,0,0,8"/>
 
             <!-- ═══ BRAND ═══ -->
             <TextBlock Text="Brand logo" Style="{StaticResource SectionHeader}" Margin="0,6,0,4"/>