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.

ViewModels/MainViewModel.Scenes.cs

@@ -76,7 +76,21 @@ public partial class MainViewModel
 
     /// <summary>True when offline — edit mode is available. When live/recording,
     /// all edit operations are locked out (gun safety).</summary>
-    public bool IsEditMode => StreamStatus == StreamStatus.Offline && !IsRecording;
+    private bool _editingActive = true;
+
+    /// <summary>The edit toggle (TopBar). On by default; going live on YouTube turns it off
+    /// and shows the live chat; stopping turns it back on.</summary>
+    public bool EditingActive
+    {
+        get => _editingActive;
+        set
+        {
+            if (SetProperty(ref _editingActive, value))
+                OnPropertyChanged(nameof(IsEditMode));
+        }
+    }
+
+    public bool IsEditMode => EditingActive;
 
     // ─── Transitions (TASK 19/23) ───