Merge Add button into DaySectionHeaderView to prevent items from being
dragged between the day header and Add button. The Add button now uses
a SwiftUI Button with its own tap handler instead of row selection.
Changes:
- Remove .addButton case from ItineraryRowItem enum
- Update DaySectionHeaderView to include Add button on the right
- Pass onAddTapped callback through configureDayHeaderCell
- Remove AddButtonRowView (no longer needed)
- Update documentation to reflect new row structure
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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