Top bar and scene strip: EDIT toggle beside REC (orange on, blue off); fixed-size centered mini screens; fixed 360px left column so the empty cell matches the layers panel

- TopBar: EDIT ToggleButton, spaced from the REC/ON-AIR segments
- MainViewModel: explicit EditingActive drives IsEditMode; YouTube go-live turns edit off, stop turns it on
- SceneThumbnailStrip: tiles fixed 196x130 and centered; dark background fills the column so its edge does not move on resize; strip row fixed 154px
- MainWindow: left column fixed at 360px; WIDTHS diagnostic log on resize

Not pushed.

Controls/TopBar.xaml

@@ -54,6 +54,21 @@
              reality line + End so nothing mid-session can be re-armed. -->
         <StackPanel Grid.Column="1" Orientation="Horizontal"
                     HorizontalAlignment="Center" VerticalAlignment="Center">
+            <!-- EDIT: explicit toggle. Blue = off (chat), orange = on (layers/properties). -->
+            <ToggleButton IsChecked="{Binding EditingActive, Mode=TwoWay}" Content="EDIT"
+                          Cursor="Hand" Height="32" Padding="14,0" Margin="0,0,24,0" VerticalAlignment="Center"
+                          FontWeight="Bold" FontSize="12" Foreground="#0b1326" BorderThickness="0">
+                <ToggleButton.Style>
+                    <Style TargetType="ToggleButton">
+                        <Setter Property="Background" Value="#38bdf8"/>
+                        <Style.Triggers>
+                            <Trigger Property="IsChecked" Value="True">
+                                <Setter Property="Background" Value="#ff8c3a"/>
+                            </Trigger>
+                        </Style.Triggers>
+                    </Style>
+                </ToggleButton.Style>
+            </ToggleButton>
             <!-- IDLE: the mode switch + the action(s) of the armed world. The three
                  controls share one height (build 77 feedback) so they read as one
                  family — segments host, Start/Go Live, and Test all sit at 32. -->