feat: product search screen with catalog add and success feedback

- Add product search screen (/products/search) as primary add flow;
  "Add" button on products list opens search, manual entry remains as fallback
- Add to shelf bottom sheet with AnimatedSwitcher success view (green checkmark)
  and SnackBar confirmation on the search screen via onAdded callback
- Manual add (AddProductScreen) shows SnackBar on success before popping back
- Extend AddProductScreen with optional nutrition fields (calories, protein,
  fat, carbs, fiber); auto-fills from catalog selection and auto-expands section
- Auto-upsert catalog product on backend when nutrition data is provided
  without a primary_product_id, linking the user product to the catalog
- Add fiber_per_100g field to CatalogProduct model and CreateRequest
- Add 16 new L10n keys across all 12 locales (addProduct, addManually,
  searchProducts, quantity, storageDays, addToShelf, nutritionOptional,
  calories, protein, fat, carbs, fiber, productAddedToShelf, etc.)

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

client/lib/l10n/app_localizations_it.dart

@@ -475,4 +475,68 @@ class AppLocalizationsIt extends AppLocalizations {
 
   @override
   String get processingProducts => 'Elaborazione...';
+
+  @override
+  String get clearAllProducts => 'Cancella tutto';
+
+  @override
+  String get clearAllConfirmTitle => 'Cancellare tutti i prodotti?';
+
+  @override
+  String get clearAllConfirmMessage =>
+      'Tutti i prodotti verranno eliminati definitivamente.';
+
+  @override
+  String get addManually => 'Manualmente';
+
+  @override
+  String get scan => 'Scansiona';
+
+  @override
+  String get addProduct => 'Aggiungi';
+
+  @override
+  String get searchProducts => 'Cerca prodotti';
+
+  @override
+  String get searchProductsHint =>
+      'Digita il nome del prodotto o aggiungi manualmente';
+
+  @override
+  String noSearchResults(String query) {
+    return 'Nessun risultato per \"$query\"';
+  }
+
+  @override
+  String get quantity => 'Quantità';
+
+  @override
+  String get storageDays => 'Giorni di conservazione';
+
+  @override
+  String get addToShelf => 'Aggiungi alla dispensa';
+
+  @override
+  String get errorGeneric => 'Qualcosa è andato storto';
+
+  @override
+  String get nutritionOptional => 'Valori nutrizionali per 100g (opzionale)';
+
+  @override
+  String get calories => 'Calorie';
+
+  @override
+  String get protein => 'Proteine';
+
+  @override
+  String get fat => 'Grassi';
+
+  @override
+  String get carbs => 'Carboidrati';
+
+  @override
+  String get fiber => 'Fibre';
+
+  @override
+  String get productAddedToShelf => 'Aggiunto alla dispensa';
 }