Sticky per-step header for multi-step flows inside a native host (`?host=…`), where the web hides its own AppHeader/AppFooter. Lays out a back control at the start, a centered title, and an optional action at the end — plus a slim segmented progress bar. Back navigation is delegated via `onBack` (omit it on the first step); top safe-area padding clears the iOS notch. RTL-aware.