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/dialogs/cross_paichan_dialog.dart

@@ -0,0 +1,126 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+
+Future<bool> showCrossPaichanDialog({
+  required BuildContext context,
+  required String currentPaicha,
+  required String newPaicha,
+}) async {
+  final dialogFocus = FocusNode();
+  var selectedIndex = 0; // 0 = "否" (default), 1 = "是"
+
+  final dialogFuture = showDialog<bool>(
+    context: context,
+    barrierDismissible: false,
+    builder: (ctx) => StatefulBuilder(
+      builder: (ctx, setDialogState) {
+        void dismissAsNo() => Navigator.of(ctx).pop(false);
+        void confirmSwitch() => Navigator.of(ctx).pop(true);
+
+        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),
+                  _DialogOptionButton(
+                    label: '否',
+                    selected: selectedIndex == 0,
+                    onTap: dismissAsNo,
+                  ),
+                  const SizedBox(height: 8),
+                  _DialogOptionButton(
+                    label: '是，切换排产号',
+                    selected: selectedIndex == 1,
+                    onTap: confirmSwitch,
+                  ),
+                ],
+              ),
+            ),
+          ),
+        );
+      },
+    ),
+  );
+
+  WidgetsBinding.instance.addPostFrameCallback((_) {
+    dialogFocus.requestFocus();
+  });
+
+  final result = await dialogFuture;
+  dialogFocus.dispose();
+  return result ?? false;
+}
+
+class _DialogOptionButton extends StatelessWidget {
+  final String label;
+  final bool selected;
+  final VoidCallback onTap;
+
+  const _DialogOptionButton({
+    required this.label,
+    required this.selected,
+    required this.onTap,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    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,
+          ),
+        ),
+      ),
+    );
+  }
+}