feat: barcode scanning for shelf add + scan screen barcode option

- Add ShelfBarcodeScanScreen: scans barcode via mobile_scanner, looks up
  product via GET /products/barcode/{barcode} (Open Food Facts fallback),
  returns CatalogProduct to caller; loading overlay while looking up;
  "Add manually" fallback in AppBar for unknown products
- Extract AddToShelfSheet to add_to_shelf_sheet.dart (was private in
  product_search_screen.dart) so both search and scan screens can reuse it
- Add barcode icon button to ProductSearchScreen AppBar → opens scanner
- Add "Scan barcode" card (📷) to ScanScreen alongside receipt and photo modes
- Rename ScanScreen title: addFromReceiptOrPhoto → scanScreenTitle
  ("Сканировать" / "Scan & Recognize") to reflect all three modes
- Add 2 L10n keys (scanScreenTitle, barcodeScanSubtitle) across all 12 locales

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

client/lib/features/scan/scan_screen.dart

@@ -4,6 +4,9 @@ import 'package:go_router/go_router.dart';
 import 'package:image_picker/image_picker.dart';
 
 import '../../l10n/app_localizations.dart';
+import '../../shared/models/product.dart';
+import '../products/add_to_shelf_sheet.dart';
+import '../products/shelf_barcode_scan_screen.dart';
 import 'recognition_service.dart';
 
 /// Entry screen — lets the user choose how to add products.
@@ -19,7 +22,7 @@ class _ScanScreenState extends ConsumerState<ScanScreen> {
   Widget build(BuildContext context) {
     final l10n = AppLocalizations.of(context)!;
     return Scaffold(
-      appBar: AppBar(title: Text(l10n.addFromReceiptOrPhoto)),
+      appBar: AppBar(title: Text(l10n.scanScreenTitle)),
       body: ListView(
         padding: const EdgeInsets.all(20),
         children: [
@@ -43,11 +46,43 @@ class _ScanScreenState extends ConsumerState<ScanScreen> {
             subtitle: l10n.photoProductsSubtitle,
             onTap: () => _pickAndRecognize(context, _Mode.products),
           ),
+          const SizedBox(height: 16),
+          _ModeCard(
+            emoji: '📷',
+            title: l10n.scanBarcode,
+            subtitle: l10n.barcodeScanSubtitle,
+            onTap: _openBarcode,
+          ),
         ],
       ),
     );
   }
 
+  void _openBarcode() {
+    final messenger = ScaffoldMessenger.of(context);
+    final addedText = AppLocalizations.of(context)!.productAddedToShelf;
+
+    Navigator.push<CatalogProduct>(
+      context,
+      MaterialPageRoute(builder: (_) => const ShelfBarcodeScanScreen()),
+    ).then((catalogProduct) {
+      if (catalogProduct != null && mounted) {
+        showModalBottomSheet<void>(
+          context: context,
+          isScrollControlled: true,
+          builder: (_) => AddToShelfSheet(
+            catalogProduct: catalogProduct,
+            onAdded: () => messenger.showSnackBar(
+              SnackBar(
+                content: Text('${catalogProduct.displayName} — $addedText'),
+              ),
+            ),
+          ),
+        );
+      }
+    });
+  }
+
   Future<void> _pickAndRecognize(
     BuildContext context,
     _Mode mode,