feat: add cross-paichan warning dialog to multi-code boxing & fix box number calculation

Multi-code boxing mode now shows a confirmation dialog when scanning a
code from a different paichan, matching the registration page pattern.
Scans are blocked while the dialog is visible.

Also removes the redundant _lastBoxNo variable. Box numbers now always
use _maxBoxNo + 1 from the backend, fixing a bug where switching to a
new paichan incorrectly carried over the previous paichan's box number.

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

lib/pages/boxing_page.dart

@@ -123,12 +123,11 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
   int? _deletingAssignedItemId;
   String? _lastScannedPaichanNo;
   String? _paichanSwitchNotice;
+  bool _crossPaichaPending = false;
 
   // 提交中
   bool _isSubmitting = false;
 
-  // 单码装箱：上次提交的箱号（用于继续添加预填）
-  int? _lastBoxNo;
   CurrentZongpaiBoxData? _editingBox;
 
   // 重复箱号
@@ -273,8 +272,8 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
     _deletingAssignedItemId = null;
     _lastScannedPaichanNo = null;
     _paichanSwitchNotice = null;
+    _crossPaichaPending = false;
     _isSubmitting = false;
-    _lastBoxNo = null;
     _editingBox = null;
     _isDuplicateBoxNo = false;
     _statusOverrideText = null;
@@ -288,6 +287,11 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
       return;
     }
 
+    if (_crossPaichaPending) {
+      _feedbackService.trigger(FeedbackEvent.alreadyCompleted);
+      return;
+    }
+
     final parsed = CodeParser.parse(result.barcode);
 
     if (parsed.type != CodeType.zongpaiNo) {
@@ -438,11 +442,7 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
           _statusOverrideDot = StatusDotColor.red;
           return;
         }
-        if (_lastBoxNo != null) {
-          _boxNoController.text = (_lastBoxNo! + 1).toString();
-        } else {
         _boxNoController.text = (_maxBoxNo + 1).toString();
-        }
         _quantityController.text = remaining.toString();
         _quantityController.selection = TextSelection(
           baseOffset: 0,
@@ -461,12 +461,10 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
           maxBoxNo: _maxBoxNo,
         );
         if (decision.isSwitched) {
-          _manyToOnePackedItems.clear();
-          _editingPackedItemId = null;
-          _editingPackedQuantity = '';
-          _deletingPackedItemId = null;
-          _paichanSwitchNotice = '排产号已切换：${_paichanNo ?? "--"}';
-          _feedbackService.trigger(FeedbackEvent.paichanSwitch);
+          _crossPaichaPending = true;
+          _feedbackService.trigger(FeedbackEvent.alreadyCompleted);
+          _showCrossPaichaDialog();
+          return;
         } else {
           _paichanSwitchNotice = null;
         }
@@ -500,6 +498,178 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
     _isDuplicateBoxNo = _boxNoIsDuplicate();
   }
 
+  // === 跨排产号确认对话框 ===
+
+  void _showCrossPaichaDialog() {
+    final currentPaicha = _lastScannedPaichanNo ?? '--';
+    final newPaicha = _paichanNo ?? '--';
+    final dialogFocus = FocusNode();
+    var dismissed = false;
+    var selectedIndex = 0; // 0 = "否" (default), 1 = "是"
+
+    void dismissAsNo() {
+      if (dismissed) return;
+      dismissed = true;
+      Navigator.of(context).pop();
+      _cancelPaichanSwitch();
+    }
+
+    void confirmSwitch() {
+      if (dismissed) return;
+      dismissed = true;
+      Navigator.of(context).pop();
+      _confirmPaichanSwitch();
+    }
+
+    showDialog(
+      context: context,
+      barrierDismissible: false,
+      builder: (ctx) => StatefulBuilder(
+        builder: (ctx, setDialogState) {
+          return KeyboardListener(
+            focusNode: dialogFocus,
+            onKeyEvent: (event) {
+              if (event is! KeyDownEvent) return;
+              if (event.logicalKey == LogicalKeyboardKey.arrowUp) {
+                setDialogState(() => selectedIndex = 0);
+              } else if (event.logicalKey == LogicalKeyboardKey.arrowDown) {
+                setDialogState(() => selectedIndex = 1);
+              } else if (event.logicalKey == LogicalKeyboardKey.enter) {
+                selectedIndex == 0 ? dismissAsNo() : confirmSwitch();
+              } else if (event.logicalKey == LogicalKeyboardKey.escape) {
+                dismissAsNo();
+              }
+            },
+            child: PopScope(
+              canPop: false,
+              onPopInvokedWithResult: (didPop, _) {
+                if (!didPop) dismissAsNo();
+              },
+              child: AlertDialog(
+                backgroundColor: Colors.orange.shade50,
+                title: Row(
+                  children: [
+                    Icon(Icons.warning_amber, color: Colors.orange.shade800),
+                    const SizedBox(width: 8),
+                    Text(
+                      '跨排产号扫描',
+                      style: TextStyle(color: Colors.orange.shade900),
+                    ),
+                  ],
+                ),
+                content: Column(
+                  mainAxisSize: MainAxisSize.min,
+                  crossAxisAlignment: CrossAxisAlignment.stretch,
+                  children: [
+                    Text(
+                      '当前排产号：$currentPaicha\n'
+                      '新排产号：$newPaicha\n\n'
+                      '是否切换到新的排产号？\n\n'
+                      '选择「是」将放弃当前已扫描的所有数据。',
+                      style: const TextStyle(fontSize: 14),
+                    ),
+                    const SizedBox(height: 16),
+                    _dialogOptionBtn(
+                      label: '否',
+                      selected: selectedIndex == 0,
+                      onTap: dismissAsNo,
+                    ),
+                    const SizedBox(height: 8),
+                    _dialogOptionBtn(
+                      label: '是，切换排产号',
+                      selected: selectedIndex == 1,
+                      onTap: confirmSwitch,
+                    ),
+                  ],
+                ),
+              ),
+            ),
+          );
+        },
+      ),
+    );
+
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      dialogFocus.requestFocus();
+    });
+  }
+
+  void _confirmPaichanSwitch() {
+    setState(() {
+      _crossPaichaPending = false;
+      _manyToOnePackedItems.clear();
+      _editingPackedItemId = null;
+      _editingPackedQuantity = '';
+      _deletingPackedItemId = null;
+      _paichanSwitchNotice = '排产号已切换：${_paichanNo ?? "--"}';
+      _boxNoController.text = (_maxBoxNo + 1).toString();
+      _lastScannedPaichanNo = _paichanNo;
+    });
+    _feedbackService.trigger(FeedbackEvent.paichanSwitch);
+
+    // Continue with remaining quantity flow
+    final remaining = _remainingQuantity;
+    if (remaining <= 0) {
+      _quantityController.clear();
+      if (_manyToOnePackedItems.isEmpty && _currentZongpaiBoxes.isNotEmpty) {
+        _completedJumpBoxNo = _currentZongpaiBoxes.first.boxNo;
+      }
+      setState(() {
+        _statusOverrideText = '该总排号已全部装箱完毕';
+        _statusOverrideDot = StatusDotColor.red;
+      });
+      return;
+    }
+    setState(() {
+      _quantityController.text = remaining.toString();
+      _quantityController.selection = TextSelection(
+        baseOffset: 0,
+        extentOffset: _quantityController.text.length,
+      );
+      _isDuplicateBoxNo = _boxNoIsDuplicate();
+    });
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      if (mounted && _mode == BoxingMode.multiCode) {
+        _quantityFocusNode.requestFocus();
+      }
+    });
+  }
+
+  void _cancelPaichanSwitch() {
+    setState(() {
+      _crossPaichaPending = false;
+      _zongpaiNo = null;
+      _paichanNo = _lastScannedPaichanNo;
+      _phase = _Phase.waiting;
+    });
+  }
+
+  Widget _dialogOptionBtn({
+    required String label,
+    required bool selected,
+    required VoidCallback onTap,
+  }) {
+    return SizedBox(
+      width: double.infinity,
+      height: 40,
+      child: ElevatedButton(
+        onPressed: onTap,
+        style: ElevatedButton.styleFrom(
+          backgroundColor:
+              selected ? Colors.blue.shade700 : Colors.grey.shade200,
+          foregroundColor: selected ? Colors.white : Colors.black87,
+          shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(8)),
+        ),
+        child: Text(
+          label,
+          style: TextStyle(
+            fontWeight: selected ? FontWeight.w800 : FontWeight.w600,
+          ),
+        ),
+      ),
+    );
+  }
+
   // === 重复箱号检测 ===
 
   bool _boxNoIsDuplicate() {
@@ -674,7 +844,6 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
     _feedbackService.trigger(FeedbackEvent.submitSuccess);
 
     setState(() {
-      _lastBoxNo = boxNo;
       if (_mode == BoxingMode.singleCode && boxItemId != null) {
         _currentZongpaiBoxes = List.from(_currentZongpaiBoxes)
           ..add(
@@ -765,7 +934,6 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
       }).toList();
       _replaceExistingBoxItem(editing, boxNo, quantity);
       _editingBox = null;
-      _lastBoxNo = _maxBoxNo;
       _boxNoController.text = (_maxBoxNo + 1).toString();
       final remaining = _remainingQuantity;
       if (remaining > 0) {
@@ -884,7 +1052,6 @@ class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
   }
 
   void _refreshSingleCodeNewInput({bool focusQuantity = true}) {
-    _lastBoxNo = _maxBoxNo;
     _boxNoController.text = (_maxBoxNo + 1).toString();
     final remaining = _remainingQuantity;
     if (remaining > 0) {