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.Streaming.Operations.cs

@@ -224,6 +224,7 @@ public partial class MainViewModel : ViewModelBase
         // before that (2026-09-25 diagnosis), so chat resolution runs AFTER the pump
         // starts. Chat is non-fatal — if it fails, the stream continues without chat.
         LiveScene = StagedScene;
+        EditingActive = false;
         await _framePump.StartAsync(); // never throws; failures log + surface via Failed
 
         string? liveChatId = null;
@@ -327,6 +328,7 @@ public partial class MainViewModel : ViewModelBase
         StreamStatus = StreamStatus.Offline;
         IsRecording = false;
         LiveScene = null;
+        EditingActive = true;
         // TASK 41: a stopped test broadcast forgets its drawer session (idempotent)
         // and drops the TEST face — a test never outlives its run.
         TestSession.EndTestSilently();