Brand logo: one locked image layer per scene above the background, image from App Settings

- Source.IsBrandLogo persisted (additive migration); layer healed on load, removal blocked
- Image stored as an asset; settings keep the asset id and file name
- App Settings: Brand Logo file field with Browse
- Default placement top-left; per-scene eye uses the existing element visibility
Test: BrandLogoTests; layout/background tests pass (34). Not pushed.

Controls/OverlayHost.xaml

@@ -56,6 +56,17 @@
                             <ComboBoxItem Content="Phone (Vertical Rectangle)" Tag="{x:Static Models:ZoomShape.VerticalRectangle}"/>
                             <ComboBoxItem Content="Tablet (Horizontal Rectangle)" Tag="{x:Static Models:ZoomShape.HorizontalRectangle}"/>
                         </ComboBox>
+                        <TextBlock Text="Brand Logo" Style="{StaticResource YtLabel}" Margin="0,12,0,4"/>
+                        <Grid Margin="0,0,0,4">
+                            <Grid.ColumnDefinitions>
+                                <ColumnDefinition Width="*"/>
+                                <ColumnDefinition Width="Auto"/>
+                            </Grid.ColumnDefinitions>
+                            <TextBox Style="{StaticResource YtTextBox}" Text="{Binding BrandLogoFileDisplay, Mode=OneWay}"
+                                     IsReadOnly="True" Foreground="#a0a0b0"/>
+                            <Button Grid.Column="1" Content="Browse…" Command="{Binding BrowseBrandLogoCommand}"
+                                    Style="{StaticResource YtButtonSecondary}" Margin="8,0,0,0"/>
+                        </Grid>
                         <TextBlock Text="Scene Transition" Style="{StaticResource YtLabel}" Margin="0,12,0,4"/>
                         <ComboBox Style="{StaticResource YtComboBox}" SelectedValue="{Binding SelectedTransitionType}" SelectedValuePath="Tag">
                             <ComboBoxItem Content="Cut" Tag="{x:Static services:TransitionType.Cut}"/>