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).

Controls/PreviewPane.xaml.cs

@@ -479,7 +479,7 @@ public partial class PreviewPane : UserControl
         // the mode where nothing is editable. Dragging it afterwards is handled by MouseMove.
         if (!vm.IsEditMode)
         {
-            vm.ToggleZoom();
+            vm.ToggleZoom(p.X, p.Y);
             e.Handled = true;
             return;
         }
@@ -619,7 +619,10 @@ public partial class PreviewPane : UserControl
             return;
         }
 
-        vm.ToggleZoom();
+        var grid = (Grid)sender;
+        var toCanvas = CanvasGrid.TransformToVisual(grid).Inverse;
+        var p = toCanvas.Transform(e.GetPosition(grid));
+        vm.ToggleZoom(p.X, p.Y);
         e.Handled = true;
     }