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).

ViewModels/MainViewModel.Zoom.cs

@@ -21,7 +21,6 @@ public partial class MainViewModel
 {
     public void ActivateZoom(double clickX, double clickY)
     {
-        if (!(IsLive || IsRecording)) return;
         var maxW = MasterFrameWidth * 0.7;
         var maxH = MasterFrameHeight * 0.7;
         double w, h;
@@ -74,9 +73,16 @@ public partial class MainViewModel
         RefreshZoomUi();
     }
 
-    public void ToggleZoom()
+    /// <summary>
+    /// The gesture behind the zoom box: activate it centred on the click, or dismiss it if it is
+    /// already open. Both halves live here because the old split (activate while live, dismiss
+    /// only otherwise) made the gesture do nothing at all when idle — the box could be turned off
+    /// but never on (creator, 2026-11-01).
+    /// </summary>
+    public void ToggleZoom(double clickX, double clickY)
     {
         if (_zoomState.IsActive) DismissZoom();
+        else ActivateZoom(clickX, clickY);
     }
 
     public void DismissZoom()