NavigationView: clamp the pane toggle button width to zero

With a PaneTitle and an open overlay pane showing the back/close button, the
toggle button width is OpenPaneLength - 40, which is negative for a small
OpenPaneLength. FrameworkElement.Width rejects negative values.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: ec9a38b2-2629-4a10-bfb5-552decfe6848

controls/dev/NavigationView/NavigationView.cpp

@@ -4655,7 +4655,8 @@ void NavigationView::UpdatePaneToggleSize()
 
             if (auto toggleButton = m_paneToggleButton.get())
             {
-                toggleButton.Width(togglePaneButtonWidth);
+                // OpenPaneLength can be smaller than the back/close button, and Width rejects negative values.
+                toggleButton.Width(std::max(0.0, togglePaneButtonWidth));
             }
         }
     }