fix: UI/UX improvements from design review

- Move FABs (timer + calculator) from bottom-left to bottom-right to prevent sidebar content overlap
- Add sidebar-scroll class with bottom padding to clear FABs
- Stronger selected recipe card highlight (green left border + tinted bg, dark mode aware)
- Add filter-tags-scroll class for hidden-scrollbar horizontal filter strips
- Auto-scroll active tab into view on mobile (useRef + scrollIntoView)
- Add detail-text class (max-width: 72ch) to usage descriptions, tips, and brief steps
- Add gradient fade hint on nav overflow edge for mobile
- Compact header brand on mobile (<640px)