feat(ui): gear to the rail, EDIT on the panel edge, preview undo/redo

Three creator directives (2026-11-01), done ahead of the surge.

**Gear moved to the preview rail** — first tab, orange (Brush.Attention), gear icon
only. Left-click opens the same app menu the top bar gear owned (Settings / Bug /
Feature / About), right-click goes straight to App Settings; both behaviours moved
with it, so nothing lost an entry point. TopBar keeps the wordmark and EDIT.

**EDIT right edge aligned to the layers panel.** The work area is margin 8 + a 360px
column, the top bar is padding 16, so the shared edge is 352 inside the bar. TopBar
column 0 is now a fixed 352 grid with EDIT right-aligned in it — alignment by
arithmetic instead of by eye.

**Preview undo/redo: Ctrl+Z / Ctrl+Shift+Z.** Models/PreviewHistory snapshots element
REFERENCES plus geometry, visibility, order and membership — a deep clone would mirror
every property of a dozen element subclasses and quietly miss the next one added.
One snapshot per gesture (pushed at drag/resize start, never in the move loop), bounded
at 50, redo survives multiple undos, cleared on layout load. Gated on EditingActive:
undo while the preview shows program output would rewrite a scene nobody is looking at.

Honest scope, in the type's doc comment: layout edits only. Colour/opacity/border/font
edits are not captured yet, so Ctrl+Z after one of those restores the last layout change.

6 tests in PreviewHistoryTests; 19 pass across it and the neighbouring UI classes; Debug
and Release both build at 0 warnings.

ViewModels/MainViewModel.cs

@@ -208,9 +208,9 @@ public partial class MainViewModel : ViewModelBase
         AddSourceCommand = new RelayCommand(parameter => AddSource(parameter));
         AddWebcamCommand = new RelayCommand(_ => _ = AddWebcamToStagedSceneAsync());
         AddImageCommand = new RelayCommand(_ => AddImage());
-        RemoveSourceCommand = new RelayCommand(element => RemoveElement(element as SceneElement));
+        RemoveSourceCommand = new RelayCommand(element => { PushPreviewUndo(); RemoveElement(element as SceneElement); });
         EditElementCommand = new RelayCommand(element => BeginEditElement(element as SceneElement));
-        ToggleElementVisibilityCommand = new RelayCommand(element => ToggleElementVisibility(element as SceneElement));
+        ToggleElementVisibilityCommand = new RelayCommand(element => { PushPreviewUndo(); ToggleElementVisibility(element as SceneElement); });
         BrowseAlertVideoCommand = new RelayCommand(element => BrowseAlertVideo(element as Source), element => element is Source { Type: SourceType.AlertBox });
         ResetAlertVideoCommand = new RelayCommand(element => ResetAlertVideo(element as Source), element => element is Source { Type: SourceType.AlertBox });
         ChangeWebcamCommand = new RelayCommand(_ => _ = ChangeWebcamAsync(), _ => CanChangeWebcam);
@@ -233,6 +233,7 @@ public partial class MainViewModel : ViewModelBase
         ToggleGameMuteCommand = new RelayCommand(_ => ToggleGameMute());
         OpenMicPickerCommand = new RelayCommand(_ => PickMicrophone());
         OpenSettingsCommand = new RelayCommand(_ => ShowOverlay(nameof(IsSettingsOpen), "App Settings"));
+        InitPreviewHistory();
         OpenBugCommand = new RelayCommand(_ => ShowOverlay(nameof(IsBugOpen), "Report Bug"));
         OpenFeatureCommand = new RelayCommand(_ => ShowOverlay(nameof(IsFeatureOpen), "Feature Request"));
         OpenAboutCommand = new RelayCommand(_ => ShowOverlay(nameof(IsAboutOpen), "About"));
@@ -450,6 +451,7 @@ public partial class MainViewModel : ViewModelBase
         {
             _isLoading = false;
         }
+        ClearPreviewHistory();
         StagedScene = Scenes.FirstOrDefault(s => SceneCatalog.Is(s.Name, SceneCatalog.Starting)) ?? Scenes.FirstOrDefault();
         UpdateActiveBackground();
         UpdateActiveChatBoxPreviews();