Configure the mobile auto-collapse target for <cngx-stepper>. Under
a narrow viewport (max-width: 480px), the classic strip swaps to
the chosen variant - 'text' (default) renders <cngx-text-stepper>,
'dots' renders <cngx-dot-stepper>, 'off' keeps the classic
strip. The Material twin <cngx-mat-stepper> ignores this setting.