refactor: split boxing page into part files for scan, submit, ops and status

Extract large methods from _BoxingPageState into 6 part files:
- boxing_scan_part.dart: scan handling and auto-scan processing
- boxing_submit_part.dart: submit/update/delete API calls
- boxing_box_mutation_part.dart: box item mutations
- boxing_single_ops_part.dart: single-code boxing operations
- boxing_multi_ops_part.dart: multi-code boxing operations
- boxing_status_part.dart: status bar management

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

lib/pages/boxing/parts/boxing_single_ops_part.dart

@@ -0,0 +1,133 @@
+// ignore_for_file: invalid_use_of_protected_member, unnecessary_this
+
+part of '../../boxing_page.dart';
+
+extension _BoxingSingleOpsPart on _BoxingPageState {
+  void _startEditAssigned(CurrentZongpaiBoxData item) {
+    setState(() {
+      _editingAssignedItemId = item.boxItemId;
+      _editingAssignedQuantity = item.quantity.toString();
+      _deletingAssignedItemId = null;
+      _statusOverrideText = null;
+      _statusOverrideDot = null;
+    });
+  }
+
+  void _cancelEditAssigned() {
+    setState(() {
+      _editingAssignedItemId = null;
+      _editingAssignedQuantity = '';
+    });
+  }
+
+  int _maxAssignedQuantity(CurrentZongpaiBoxData item) {
+    return boxing_calculations.maxAssignedQuantity(
+      totalQuantity: _erpQuantity,
+      packedQuantity: _packedQuantity,
+      itemQuantity: item.quantity,
+    );
+  }
+
+  void _refreshSingleCodeNewInput({bool focusQuantity = true}) {
+    _boxNoController.text = (_maxBoxNo + 1).toString();
+    final remaining = _remainingQuantity;
+    if (remaining > 0) {
+      _quantityController.text = remaining.toString();
+      _quantityController.selection = TextSelection(
+        baseOffset: 0,
+        extentOffset: _quantityController.text.length,
+      );
+      if (focusQuantity) {
+        _focusQuantityInput();
+      }
+    } else {
+      _quantityController.clear();
+    }
+    _isDuplicateBoxNo = false;
+  }
+
+  Future<void> _saveAssignedItem(CurrentZongpaiBoxData item) async {
+    final quantity = int.tryParse(_editingAssignedQuantity);
+    if (quantity == null || quantity <= 0) {
+      this._showStatusOverride(
+        '请输入有效数量',
+        StatusDotColor.red,
+        const Duration(seconds: 2),
+      );
+      return;
+    }
+    if (quantity > this._maxAssignedQuantity(item)) {
+      this._showStatusOverride(
+        '超出可装数量上限',
+        StatusDotColor.red,
+        const Duration(seconds: 2),
+      );
+      return;
+    }
+
+    setState(() => _isSubmitting = true);
+    final action = await _apiActions.updateBoxRecord(
+      boxItemId: item.boxItemId,
+      boxNo: item.boxNo,
+      quantity: quantity,
+    );
+    if (!mounted) return;
+
+    setState(() => _isSubmitting = false);
+    if (action.success) {
+      setState(() {
+        _currentZongpaiBoxes = _currentZongpaiBoxes.map((box) {
+          if (box.boxItemId != item.boxItemId) return box;
+          return CurrentZongpaiBoxData(
+            boxItemId: box.boxItemId,
+            boxNo: box.boxNo,
+            quantity: quantity,
+          );
+        }).toList();
+        this._replaceExistingBoxItem(item, item.boxNo, quantity);
+        _editingAssignedItemId = null;
+        _editingAssignedQuantity = '';
+        this._refreshSingleCodeNewInput();
+      });
+      this._showStatusOverride(
+        '修改成功',
+        StatusDotColor.green,
+        const Duration(milliseconds: 1500),
+      );
+    } else {
+      this._showActionError(action, fallback: '修改失败');
+    }
+  }
+
+  Future<void> _deleteAssignedItem(CurrentZongpaiBoxData item) async {
+    setState(() => _isSubmitting = true);
+    final action = await _apiActions.deleteBoxRecord(boxItemId: item.boxItemId);
+    if (!mounted) return;
+
+    setState(() {
+      _isSubmitting = false;
+      if (action.success) {
+        _currentZongpaiBoxes = _currentZongpaiBoxes
+            .where((box) => box.boxItemId != item.boxItemId)
+            .toList();
+        this._removeExistingBoxItem(item.boxItemId);
+        if (_editingAssignedItemId == item.boxItemId) {
+          _editingAssignedItemId = null;
+          _editingAssignedQuantity = '';
+        }
+        _deletingAssignedItemId = null;
+        this._refreshSingleCodeNewInput();
+      }
+    });
+
+    if (action.success) {
+      this._showStatusOverride(
+        '删除成功',
+        StatusDotColor.green,
+        const Duration(milliseconds: 1500),
+      );
+    } else {
+      this._showActionError(action, fallback: '删除失败');
+    }
+  }
+}