refactor: extract boxing page into modular components

Split the monolithic boxing_page.dart into separate files:
- boxing_models.dart: data models (_ManyToOnePackedItem, _Phase)
- widgets/boxing_shared_widgets.dart: shared UI components
- widgets/single_code_body.dart: single-code boxing UI
- widgets/multi_code_body.dart: multi-code boxing UI
- widgets/multi_packed_row.dart: multi-code packed row widget
- widgets/single_assigned_row.dart: single-code assigned row widget

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/pages/boxing/boxing_models.dart

@@ -0,0 +1,35 @@
+class ManyToOnePackedItem {
+  final int boxItemId;
+  final String zongpaiNo;
+  final String? paichanNo;
+  final String? workOrderNo;
+  final int boxNo;
+  final int quantity;
+  final int? totalQuantity;
+
+  const ManyToOnePackedItem({
+    required this.boxItemId,
+    required this.zongpaiNo,
+    this.paichanNo,
+    required this.workOrderNo,
+    required this.boxNo,
+    required this.quantity,
+    this.totalQuantity,
+  });
+
+  ManyToOnePackedItem copyWith({
+    int? boxNo,
+    int? quantity,
+    int? totalQuantity,
+  }) {
+    return ManyToOnePackedItem(
+      boxItemId: boxItemId,
+      zongpaiNo: zongpaiNo,
+      paichanNo: paichanNo,
+      workOrderNo: workOrderNo,
+      boxNo: boxNo ?? this.boxNo,
+      quantity: quantity ?? this.quantity,
+      totalQuantity: totalQuantity ?? this.totalQuantity,
+    );
+  }
+}