feat: extend boxing page for accessory boxing support

Add pending accessories section to both single-code and multi-code boxing
modes. Accessories display with orange styling and a "加入本箱" button that
calls saveBoxRecord with item_type=accessory. Extend API service with
PendingAccessory model, itemType fields on data classes, and optional
accessory_id parameter on saveBoxRecord. Show accessories with visual
indicator (wrench icon + orange background) in boxing detail page.

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

lib/pages/boxing_page.dart

@@ -107,6 +107,9 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
   // 多码凑箱：已完成装箱时可跳转的箱号
   int? _completedJumpBoxNo;
 
+  // 待装箱附件
+  List<PendingAccessory> _pendingAccessories = [];
+
   @override
   void initState() {
     super.initState();
@@ -244,6 +247,7 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
     _isDuplicateBoxNo = false;
     _statusOverrideText = null;
     _statusOverrideDot = null;
+    _pendingAccessories = [];
   }
 
   // === 重复箱号检测 ===
@@ -415,6 +419,9 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
                           this._deletePackedItem(item),
                       onCancelDeletePackedItem: () =>
                           setState(() => _deletingPackedItemId = null),
+                      pendingAccessories: _pendingAccessories,
+                      onSubmitAccessory: (accessory) =>
+                          this._submitAccessory(accessory),
                     )
                   : SingleCodeBody(
                       isWaiting:
@@ -465,6 +472,9 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
                           this._deleteAssignedItem(item),
                       onCancelDeleteAssigned: () =>
                           setState(() => _deletingAssignedItemId = null),
+                      pendingAccessories: _pendingAccessories,
+                      onSubmitAccessory: (accessory) =>
+                          this._submitAccessory(accessory),
                     ),
             ),