- Implemented ScrollOffsetPreferenceKey for BookmarkDetailView2
- Replaced onScrollGeometryChange + onScrollPhaseChange with onPreferenceChange
- Removed currentScrollOffset and scrollViewHeight state variables
- Changed jumpButton from var to func with containerHeight parameter
- Fixed excessive spacing between header and content by using ZStack layout
Layout fix: Header image is now in ZStack background with content in foreground,
eliminating double spacing that occurred with separate VStacks.
Performance: Same PreferenceKey benefits as LegacyView - more efficient scroll tracking.