feat(37): expose the staged-scene factory reset

App Settings can now restore only the staged canonical scene. The VM saves
pending edits first, resets that scene, reloads immediately, and keeps the same
scene staged. Other scene edits survive; the action is disabled while live or
recording. 615 tests green, clean build with 0 warnings.

Reference: OBS Scene Collections keep scene/source sets distinct:
https://obsproject.com/kb/scene-collections

Controls/SettingsDrawer.xaml

@@ -127,6 +127,10 @@
             <TextBlock Text="Layout defaults" Style="{StaticResource SectionHeader}" Margin="0,10,0,4"/>
             <TextBlock Text="Replaces every scene and layer with this build’s factory layout. Your selected webcam stays the same."
                        Style="{StaticResource PropertyHint}" Margin="10,0,0,6"/>
+            <Button Content="Restore staged scene" Command="{Binding RestoreStagedSceneDefaultsCommand}"
+                    Style="{StaticResource YtButtonSecondary}" Margin="10,0,0,4" HorizontalAlignment="Left"/>
+            <TextBlock Text="Resets only the scene currently staged in the preview."
+                       Style="{StaticResource PropertyHint}" Margin="10,0,0,8"/>
             <Button Content="Restore all scenes" Command="{Binding RestoreAllLayoutDefaultsCommand}"
                     Style="{StaticResource YtButtonSecondary}" Margin="10,0,0,10" HorizontalAlignment="Left"/>