Both backends share one SelectionState owned by MainWindow and live
side by side in a QStackedWidget — switching is setCurrentWidget, so
no widget is destroyed mid-session (tearing a VTK window down leaves
dangling make-current callbacks). The choice persists in QSettings.
CanvasCapabilities declares per-backend gaps (force diagrams and
video export stay PyVista-only, both are documented and greyed out)
and the UI gates on capabilities rather than the backend name. The
architecture gate now allows the canvas_plotly package.
