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_ko.arb

@@ -100,6 +100,8 @@
   "navProducts": "식품",
   "navRecipes": "레시피",
   "addFromReceiptOrPhoto": "영수증 또는 사진으로 추가",
+  "scanScreenTitle": "스캔 및 인식",
+  "barcodeScanSubtitle": "바코드로 제품 찾기",
   "chooseMethod": "방법 선택",
   "photoReceipt": "영수증 촬영",
   "photoReceiptSubtitle": "영수증의 모든 상품 인식",