fix(zoom): the gesture now creates the box instead of only closing it

Creator report: Ctrl+right in edit mode and left-click in preview did nothing when
neither streaming nor recording. The bindings were correct; the action was not.

Two causes, both in the model rather than the handler:
- ActivateZoom opened with `if (!(IsLive || IsRecording)) return;` — the box could not be
  created outside a session at all.
- ToggleZoom only ever called DismissZoom, so "toggle" could turn it off and never on.

Now ToggleZoom(x, y) activates at the cursor or dismisses if already open, and the live-only
guard is gone — the magnifier is an editing aid as much as a live one. Both call sites pass
the canvas point, so the box still anchors on the click rather than the frame centre.

Debug 0 warnings; ZoomBox + ZoomMagnifier tests pass (3).

docs/ai/ui-surface.md

@@ -104,7 +104,7 @@ The app is a **director's control surface**, not an editor. This replaces the OB
 
 - **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 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 gestures (2026-11-01):** *edit mode* — left selects/drags/resizes, right opens the context menu, **Ctrl+right toggles the zoom box**. *Preview mode* — **left toggles the zoom box**, right opens the context menu; **Ctrl+right does nothing at all** (no chord while live or recording — one hand, always). ⚠️ **Both the Down and the Up event must be handled for a Ctrl+right-click**: WPF's `PopupControlService` opens a context menu on right-button-**up**, so suppressing only the down event leaves the menu appearing under the gesture (found 2026-11-01 in edit mode). Menus exist on the webcam layer and the backdrop; every other layer right-clicks through to the backdrop menu. Left-dragging with the zoom box open moves it, in both modes. The social bar consumes its own click (toggle top ⇄ bottom), and the preview's tunnelling handler steps aside for it — otherwise the bar is unclickable in preview mode and drags the layer beneath it in edit mode.
+- **Preview gestures (2026-11-01):** *edit mode* — left selects/drags/resizes, right opens the context menu, **Ctrl+right toggles the zoom box**. *Preview mode* — **left toggles the zoom box**, right opens the context menu; **Ctrl+right does nothing at all** (no chord while live or recording — one hand, always). ⚠️ **Both the Down and the Up event must be handled for a Ctrl+right-click**: WPF's `PopupControlService` opens a context menu on right-button-**up**, so suppressing only the down event leaves the menu appearing under the gesture (found 2026-11-01 in edit mode). Menus exist on the webcam layer and the backdrop; every other layer right-clicks through to the backdrop menu. Left-dragging with the zoom box open moves it, in both modes. **The gesture activates the box at the cursor**, not merely dismisses it: `ActivateZoom` no longer refuses when idle (it used to return unless `IsLive || IsRecording`, and `ToggleZoom` only ever dismissed — so the gesture did nothing at all outside a session until 2026-11-01). The social bar consumes its own click (toggle top ⇄ bottom), and the preview's tunnelling handler steps aside for it — otherwise the bar is unclickable in preview mode and drags the layer beneath it in edit mode.
 - **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.