feat(itinerary): reorder Add button after day header + comprehensive docs

- Move Add button to appear immediately after Day header (before games)
- Split games out of dayHeader into separate row for correct ordering
- Add 600+ lines of inline documentation to ItineraryTableViewController
- Document architecture decisions, data flow, constraints, and algorithms
- Add function-level comments explaining drag/drop, sortOrder calculation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

SportsTime/Features/Trip/Views/TripDetailView.swift

@@ -837,14 +837,14 @@ struct TripDetailView: View {
 
             // Custom items for this day (sorted by sortOrder)
             if allowCustomItems {
+                // Add button first - always right after day header
+                sections.append(.addButton(day: dayNum))
+
                 let dayItems = customItems.filter { $0.day == dayNum }
                     .sorted { $0.sortOrder < $1.sortOrder }
                 for item in dayItems {
                     sections.append(.customItem(item))
                 }
-
-                // One add button per day
-                sections.append(.addButton(day: dayNum))
             }
         }