refactor: extract boxing logic into dedicated modules with tests

- boxing_models.dart: add BoxingMode, BoxingPhase, BoxingPageArguments
- boxing_box_mutations.dart: box item add/replace/remove operations
- boxing_calculations.dart: pure calculation helpers (quantities, validation)
- boxing_status_presenter.dart: status bar text/dot determination
- dialogs/cross_paichan_dialog.dart: cross-paichan confirmation dialog
- widgets/boxing_notice_banners.dart: notice banner widgets
- Add unit tests for mutations, calculations, and status presenter

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

lib/pages/boxing/boxing_box_mutations.dart

@@ -0,0 +1,135 @@
+import 'package:pad_scanner/pages/boxing/boxing_models.dart';
+import 'package:pad_scanner/services/api_service.dart';
+
+class BoxMutationResult {
+  final List<BoxDetailData> boxes;
+  final int maxBoxNo;
+
+  const BoxMutationResult({required this.boxes, required this.maxBoxNo});
+}
+
+int maxBoxNoFor(List<BoxDetailData> boxes) {
+  return boxes.fold<int>(0, (max, box) => box.boxNo > max ? box.boxNo : max);
+}
+
+BoxMutationResult addExistingBoxItem({
+  required List<BoxDetailData> existingBoxes,
+  required int boxNo,
+  required int quantity,
+  required int? boxItemId,
+  required String zongpaiNo,
+  required String? workOrderNo,
+  required int? totalQuantity,
+}) {
+  final item = BoxItemData(
+    boxItemId: boxItemId,
+    zongpaiNo: zongpaiNo,
+    workOrderNo: workOrderNo,
+    quantity: quantity,
+    totalQuantity: totalQuantity,
+  );
+  final index = existingBoxes.indexWhere((box) => box.boxNo == boxNo);
+  if (index < 0) {
+    final boxes = [
+      ...existingBoxes,
+      BoxDetailData(boxNo: boxNo, items: [item]),
+    ]..sort((a, b) => a.boxNo.compareTo(b.boxNo));
+    return BoxMutationResult(boxes: boxes, maxBoxNo: maxBoxNoFor(boxes));
+  }
+
+  final boxes = List<BoxDetailData>.from(existingBoxes);
+  final box = boxes[index];
+  boxes[index] = BoxDetailData(boxNo: box.boxNo, items: [...box.items, item]);
+  return BoxMutationResult(boxes: boxes, maxBoxNo: maxBoxNoFor(boxes));
+}
+
+BoxMutationResult replaceExistingBoxItem({
+  required List<BoxDetailData> existingBoxes,
+  required CurrentZongpaiBoxData editing,
+  required int boxNo,
+  required int quantity,
+  required String zongpaiNo,
+  required String? workOrderNo,
+  required int? totalQuantity,
+}) {
+  final updatedItem = BoxItemData(
+    boxItemId: editing.boxItemId,
+    zongpaiNo: zongpaiNo,
+    workOrderNo: workOrderNo,
+    quantity: quantity,
+    totalQuantity: totalQuantity,
+  );
+  return _replaceItemById(
+    existingBoxes: existingBoxes,
+    boxItemId: editing.boxItemId,
+    targetBoxNo: boxNo,
+    updatedItem: updatedItem,
+  );
+}
+
+BoxMutationResult replaceExistingPackedItem({
+  required List<BoxDetailData> existingBoxes,
+  required ManyToOnePackedItem item,
+  required int boxNo,
+  required int quantity,
+}) {
+  final updatedItem = BoxItemData(
+    boxItemId: item.boxItemId,
+    zongpaiNo: item.zongpaiNo,
+    workOrderNo: item.workOrderNo,
+    quantity: quantity,
+    totalQuantity: item.totalQuantity,
+  );
+  return _replaceItemById(
+    existingBoxes: existingBoxes,
+    boxItemId: item.boxItemId,
+    targetBoxNo: boxNo,
+    updatedItem: updatedItem,
+  );
+}
+
+BoxMutationResult removeExistingBoxItem({
+  required List<BoxDetailData> existingBoxes,
+  required int boxItemId,
+}) {
+  final boxes = <BoxDetailData>[];
+  for (final box in existingBoxes) {
+    final items = box.items
+        .where((item) => item.boxItemId != boxItemId)
+        .toList();
+    if (items.isNotEmpty) {
+      boxes.add(BoxDetailData(boxNo: box.boxNo, items: items));
+    }
+  }
+  return BoxMutationResult(boxes: boxes, maxBoxNo: maxBoxNoFor(boxes));
+}
+
+BoxMutationResult _replaceItemById({
+  required List<BoxDetailData> existingBoxes,
+  required int boxItemId,
+  required int targetBoxNo,
+  required BoxItemData updatedItem,
+}) {
+  final boxes = <BoxDetailData>[];
+  for (final box in existingBoxes) {
+    final items = box.items.where((item) {
+      return item.boxItemId != boxItemId;
+    }).toList();
+    if (items.isNotEmpty) {
+      boxes.add(BoxDetailData(boxNo: box.boxNo, items: items));
+    }
+  }
+
+  final targetIndex = boxes.indexWhere((box) => box.boxNo == targetBoxNo);
+  if (targetIndex >= 0) {
+    final target = boxes[targetIndex];
+    boxes[targetIndex] = BoxDetailData(
+      boxNo: target.boxNo,
+      items: [...target.items, updatedItem],
+    );
+  } else {
+    boxes.add(BoxDetailData(boxNo: targetBoxNo, items: [updatedItem]));
+  }
+  boxes.sort((a, b) => a.boxNo.compareTo(b.boxNo));
+  return BoxMutationResult(boxes: boxes, maxBoxNo: maxBoxNoFor(boxes));
+}