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_models.dart

@@ -1,3 +1,24 @@
+enum BoxingMode {
+  singleCode, // 单码装箱
+  multiCode, // 多码凑箱
+}
+
+enum BoxingPhase {
+  waiting, // 等待扫码
+  scanned, // 已扫码，显示信息
+  submitted, // 已提交成功
+}
+
+class BoxingPageArguments {
+  final BoxingMode initialMode;
+  final List<String> autoScanCodes;
+
+  const BoxingPageArguments({
+    required this.initialMode,
+    required this.autoScanCodes,
+  });
+}
+
 class ManyToOnePackedItem {
   final int boxItemId;
   final String zongpaiNo;