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_de.arb

@@ -100,6 +100,8 @@
   "navProducts": "Produkte",
   "navRecipes": "Rezepte",
   "addFromReceiptOrPhoto": "Aus Kassenbon oder Foto hinzufügen",
+  "scanScreenTitle": "Scannen & Erkennen",
+  "barcodeScanSubtitle": "Produkt per Barcode finden",
   "chooseMethod": "Methode wählen",
   "photoReceipt": "Kassenbon fotografieren",
   "photoReceiptSubtitle": "Alle Produkte vom Kassenbon erkennen",