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.

docs/ai/ui-surface.md

@@ -103,7 +103,8 @@ The app is a **director's control surface**, not an editor. This replaces the OB
 **Current layout (2026-10-26 redesign commits):** `MainWindow` has a top bar and health banner, then a two-column work area with a fixed 360px left column and preview column. Its top row holds stream readouts/resolution on the left and the five-scene strip above the preview; the lower row holds the left panel and central preview. The bottom bar is now only a height-preserving spacer.
 
 - **Left panel:** `EditingActive` is an explicit `EDIT` toggle, not a mode inferred from `StreamStatus`. On = scene layers + properties; off = YouTube chat. Go Live turns editing off and stopping restores it; recording does not automatically switch it off. The staged scene is the preview/edit target; `LiveScene` is the program output.
-- **Top bar:** the `EDIT` toggle sits beside the REC/ON-AIR session controls; the brand/gear and account surfaces remain at the sides. The gear's left-click menu remains; right-click opens App Settings directly.
+- **Top bar:** the `EDIT` toggle sits beside the REC/ON-AIR session controls; the brand and account surfaces remain at the sides. Its **right edge aligns with the layers panel below** (`TopBar` column 0 is a fixed **352** = work-area margin 8 + 360px column − the bar's own 16px padding), so the button and the panel share an edge. The **settings gear moved to the preview rail** (2026-11-01): first tab, orange (`Brush.Attention`), gear icon only — left-click opens the app menu (Settings / Bug / Feature / About), right-click goes straight to App Settings.
+- **Preview undo/redo (2026-11-01):** `Ctrl+Z` / `Ctrl+Shift+Z`, window-level bindings, live only while `EditingActive` — undo with the preview showing program output would rewrite a scene nobody is looking at. `Models/PreviewHistory` snapshots element **references** plus geometry, visibility, order and membership, so move / resize / reorder / add / remove / hide all undo in one step per gesture (`PushPreviewUndo` is called once at drag or resize start, never inside the move loop). Property edits (colour, opacity, border, font) are **not** captured yet — stated in the type's doc comment so nobody assumes otherwise. The stack clears on layout load. Guard: `ytLive.Tests/PreviewHistoryTests.cs`.
 - **Developer affordances are compiled out of release, not hidden.** Any control that must never ship (today: the all-resolutions override) lives in its own `UserControl` whose `Page` and `Compile` items are removed by `ytLive.csproj` when `$(Configuration) != 'Debug'`, and is added to its host at runtime inside `#if DEBUG` (`OverlayHost`). A collapsed row still exists in the visual tree of every shipped copy; "not compiled" is the only claim worth making. Label such rows with `Brush.Attention` (orange) while they are visible in Debug. Guard: `ytLive.Tests/OutputProfileTests.cs` asserts the build rule, and `Helpers/DevBuild.cs` is the single source for "is this a dev build".
 - **Scene strip:** five fixed-size cards above the preview show the scene name and hotkey; the staged scene carries a `PROGRAM` badge, and staged/live borders use orange. Click stages a scene while idle; while streaming or recording it changes program output and starts the selected transition.
 - **Preview rail:** stream info, YPP, Socials, and TEST are icon/color tabs on the preview's right edge. Socials opens an animated in-preview drawer; it is no longer a standalone modal window. The bottom live-controls row retains TRAX and the game/mic meters and controls.