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.Sources.cs

@@ -18,6 +18,7 @@ public partial class MainViewModel
     {
         var scene = StagedScene;
         if (scene == null) return;
+        PushPreviewUndo();
 
         var sourceType = parameter is SourceType typed
             ? typed