feat(37): expose the all-scenes factory reset

App Settings now offers a clearly described all-scenes reset. It is disabled
while live or recording and reloads the restored layout immediately. The
integration test drives the real VM command and checks the Settings binding.
614 tests green, clean build with 0 warnings.

Reference: OBS Scene Collections are a distinct saved set of scenes and sources:
https://obsproject.com/kb/scene-collections

Controls/SettingsDrawer.xaml

@@ -123,6 +123,13 @@
                         Value="{Binding TransitionDurationMs, Mode=TwoWay}" VerticalAlignment="Center"/>
             </DockPanel>
 
+            <!-- ═══ LAYOUT DEFAULTS (TASK 37) ═══ -->
+            <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 all scenes" Command="{Binding RestoreAllLayoutDefaultsCommand}"
+                    Style="{StaticResource YtButtonSecondary}" Margin="10,0,0,10" HorizontalAlignment="Left"/>
+
             <!-- Development panel host. Empty in a Release build: the panel itself is excluded
                  from the Release compile, and the code-behind only fills this inside #if DEBUG. -->
             <StackPanel x:Name="DeveloperPanelHost"/>