refactor(itinerary): embed Add button in day header row

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>

SportsTime/Features/Trip/Views/ItineraryTableViewWrapper.swift

@@ -159,10 +159,8 @@ struct ItineraryTableViewWrapper<HeaderContent: View>: UIViewControllerRepresent
             // Travel before this day (travel is stored on the destination day)
             let travelBefore: TravelSegment? = travelByDay[dayNum]
 
-            // ONE Add button per day - always first after day header
-            items.append(ItineraryRowItem.addButton(day: dayNum))
-
             // Custom items for this day - simply filter by day and sort by sortOrder
+            // Note: Add button is now embedded in the day header row (not a separate item)
             let dayItems = customItems.filter { $0.day == dayNum }
                 .sorted { $0.sortOrder < $1.sortOrder }