feat(mobile): Hide navigation on scroll for better mobile UX

Commit notes

- Add useScrollVisibility hook to track scroll direction on mobile - Hide mobile bottom navigation when scrolling down - Show navigation when scrolling up or at top of page - Respects prefers-reduced-motion for accessibility - Uses performant transform animation with proper easing - Auto-shows navigation when user menu is open

Co-authored-by: armin.naimi <[email redacted]>