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.

MainWindow.xaml.cs

@@ -20,6 +20,8 @@ public partial class MainWindow : Window
     {
         AppLog.Write("MainWindow ctor: before InitializeComponent");
         InitializeComponent();
+        SizeChanged += (_, e) => Helpers.AppLog.Write($"WIDTHS window={e.NewSize.Width:F0} "
+            + $"layers={LeftPanel.ActualWidth:F0} strip={SceneStrip.ActualWidth:F0} preview={PreviewPane.ActualWidth:F0}");
         AppLog.Write("MainWindow ctor: after InitializeComponent");
         _viewModel = (MainViewModel)DataContext;
         _viewModel.PropertyChanged += OnViewModelPropertyChanged;