feat: add pre-check to block batch operations when scanned items

already have location bindings (on_shelf or transferred)

- Replace single on_shelf pre-check with unified conflict detection
  for both normal shelf and transit targets
- Show conflict dialog listing duplicate and/or transferred items
  with details before any API calls are made
- Prevents partial batch failures where some items succeed before
  a conflicting item is encountered

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

lib/pages/registration_page.dart

@@ -224,16 +224,33 @@ class _RegistrationPageState extends State<RegistrationPage> {
 
   bool get _isTransitTarget => isTransitTarget(_locationType, _locationCode);
 
-  /// Find the first scanned item that is already on shelf (for batch submit blocking).
-  PaichaOverviewItem? _findOnShelfInScanned() {
-    if (_overview?.success != true) return null;
+  /// Find all scanned items that are already on shelf.
+  List<PaichaOverviewItem> _findOnShelfItemsInScanned() {
+    if (_overview?.success != true) return [];
     final scannedSet = _zongpaiNos.toSet();
-    for (final item in _overview!.items) {
-      if (scannedSet.contains(item.zongpaiNo) && item.status == 'on_shelf') {
-        return item;
-      }
-    }
-    return null;
+    return _overview!.items
+        .where((item) =>
+            scannedSet.contains(item.zongpaiNo) && item.status == 'on_shelf')
+        .toList();
+  }
+
+  /// Find all scanned items that are already transferred.
+  List<PaichaOverviewItem> _findTransferredItemsInScanned() {
+    if (_overview?.success != true) return [];
+    final scannedSet = _zongpaiNos.toSet();
+    return _overview!.items
+        .where((item) =>
+            scannedSet.contains(item.zongpaiNo) && item.status == 'transferred')
+        .toList();
+  }
+
+  /// Whether any scanned item already has a location binding (on_shelf or transferred).
+  bool _hasAnyLocatedInScanned() {
+    if (_overview?.success != true) return false;
+    final scannedSet = _zongpaiNos.toSet();
+    return _overview!.items.any((item) =>
+        scannedSet.contains(item.zongpaiNo) &&
+        (item.status == 'on_shelf' || item.status == 'transferred'));
   }
 
   Future<String?> _baseUrl() async {
@@ -311,6 +328,21 @@ class _RegistrationPageState extends State<RegistrationPage> {
 
     setState(() => _isSubmitting = true);
 
+    // For transit target: block if any scanned item already has a location
+    if (_isTransitTarget) {
+      if (_hasAnyLocatedInScanned()) {
+        final onShelfItems = _findOnShelfItemsInScanned();
+        final transferredItems = _findTransferredItemsInScanned();
+        setState(() => _isSubmitting = false);
+        _feedbackService.trigger(FeedbackEvent.submitFailure);
+        _showLocationConflictDialog(
+          onShelfItems: onShelfItems,
+          transferredItems: transferredItems,
+        );
+        return;
+      }
+    }
+
     if (_isLocked && _zongpaiNos.length > 1) {
       if (_isTransitTarget && !await _ensureBatchSamePaicha(baseUrl)) {
         if (!mounted) return;
@@ -323,15 +355,16 @@ class _RegistrationPageState extends State<RegistrationPage> {
         );
         return;
       }
-      // Pre-check: block entire batch if any item is already on shelf (normal shelf)
+      // Pre-check: block entire batch if any item already has a location binding
       if (!_isTransitTarget) {
-        final onShelfItem = _findOnShelfInScanned();
-        if (onShelfItem != null) {
+        final onShelfItems = _findOnShelfItemsInScanned();
+        final transferredItems = _findTransferredItemsInScanned();
+        if (onShelfItems.isNotEmpty || transferredItems.isNotEmpty) {
           setState(() => _isSubmitting = false);
           _feedbackService.trigger(FeedbackEvent.duplicateError);
-          _showDuplicateDialog(
-            onShelfItem.zongpaiNo,
-            {'location_code': onShelfItem.locationCode ?? '未知'},
+          _showLocationConflictDialog(
+            onShelfItems: onShelfItems,
+            transferredItems: transferredItems,
           );
           return;
         }
@@ -633,6 +666,92 @@ class _RegistrationPageState extends State<RegistrationPage> {
     );
   }
 
+  void _showLocationConflictDialog({
+    required List<PaichaOverviewItem> onShelfItems,
+    required List<PaichaOverviewItem> transferredItems,
+  }) {
+    final contentParts = <Widget>[];
+
+    if (onShelfItems.isNotEmpty) {
+      contentParts.add(
+        Text(
+          '重复上架',
+          style: TextStyle(
+            fontWeight: FontWeight.bold,
+            color: Colors.red.shade700,
+          ),
+        ),
+      );
+      for (final item in onShelfItems) {
+        contentParts.addAll([
+          Text('总排号：${item.zongpaiNo}'),
+          Text('已登记货位：${item.locationCode ?? "未知"}'),
+          const SizedBox(height: 6),
+        ]);
+      }
+    }
+
+    if (transferredItems.isNotEmpty) {
+      if (onShelfItems.isNotEmpty) {
+        contentParts.add(const Divider());
+        contentParts.add(const SizedBox(height: 4));
+      }
+      contentParts.add(
+        Text(
+          '货物已转运',
+          style: TextStyle(
+            fontWeight: FontWeight.bold,
+            color: Colors.orange.shade700,
+          ),
+        ),
+      );
+      for (final item in transferredItems) {
+        contentParts.addAll([
+          Text('总排号：${item.zongpaiNo}'),
+          Text('转运货位：${item.locationCode ?? "未知"}'),
+          const SizedBox(height: 6),
+        ]);
+      }
+    }
+
+    contentParts.add(const SizedBox(height: 4));
+    contentParts.add(
+      const Text(
+        '请核查实物，确认是否操作错误。',
+        style: TextStyle(fontWeight: FontWeight.bold),
+      ),
+    );
+
+    showDialog(
+      context: context,
+      barrierDismissible: false,
+      builder: (ctx) => AlertDialog(
+        backgroundColor: Colors.red.shade50,
+        title: const Row(
+          children: [
+            Icon(Icons.warning, color: Colors.red),
+            SizedBox(width: 8),
+            Text('操作冲突', style: TextStyle(color: Colors.red)),
+          ],
+        ),
+        content: Column(
+          mainAxisSize: MainAxisSize.min,
+          crossAxisAlignment: CrossAxisAlignment.start,
+          children: contentParts,
+        ),
+        actions: [
+          TextButton(
+            onPressed: () {
+              _feedbackService.stopAlert();
+              Navigator.pop(ctx);
+            },
+            child: const Text('关闭'),
+          ),
+        ],
+      ),
+    );
+  }
+
   void _showCrossPaichaDialog(String newZongpaiNo) {
     final currentPaicha = _overview?.paichaNo ?? '--';
     final dialogFocus = FocusNode();