feat(alerts): a Test alert button in the alert properties

Sits to the right of the Built-in video pill, labelled "Test alert". It enqueues one synthetic
Super Chat through the real AlertOverlayLayer, so what plays is exactly what a viewer triggers —
the custom clip when one is set, the built-in clip otherwise — and it goes through the same
output path (preview and encoder) as a live alert. Local only: nothing reaches YouTube and no
chat row is invented.

Controls/LeftPanel.xaml

@@ -767,6 +767,13 @@
                                            Foreground="#888" FontSize="10" TextWrapping="Wrap" Margin="0,0,0,6"/>
 
                                 <DockPanel Margin="0,0,0,4">
+                                    <!-- Test alert (creator ruling 2026-11-01): plays the clip this box
+                                         would play — custom if one is set, built-in otherwise. -->
+                                    <Button DockPanel.Dock="Right" Content="▶  Test alert"
+                                            Command="{Binding TestAlertCommand}"
+                                            Style="{StaticResource YtButtonSecondary}"
+                                            Padding="8,2" FontSize="11" VerticalAlignment="Center"
+                                            ToolTip="Play the alert here, exactly as a viewer would trigger it"/>
                                     <TextBlock DockPanel.Dock="Left" Text="Built-in video" Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11" VerticalAlignment="Center" Width="84"/>
                                     <ToggleButton Style="{StaticResource PillToggle}"
                                                   IsChecked="{Binding SelectedElement.AlertUseDefaultVideo, Mode=TwoWay}"