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/l10n/app_localizations_de.dart

@@ -291,6 +291,12 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get addFromReceiptOrPhoto => 'Aus Kassenbon oder Foto hinzufügen';
 
+  @override
+  String get scanScreenTitle => 'Scannen & Erkennen';
+
+  @override
+  String get barcodeScanSubtitle => 'Produkt per Barcode finden';
+
   @override
   String get chooseMethod => 'Methode wählen';