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>

backend/cmd/server/init.go

@@ -45,7 +45,7 @@ func initApp(appConfig *config.Config, pool *pgxpool.Pool) (*App, error) {
 	productRepository := product.NewRepository(pool)
 	openFoodFactsClient := product.NewOpenFoodFacts()
 	productHandler := product.NewHandler(productRepository, openFoodFactsClient)
-	userProductHandler := userproduct.NewHandler(userProductRepository)
+	userProductHandler := userproduct.NewHandler(userProductRepository, productRepository)
 
 	// Kafka producer
 	kafkaProducer, kafkaProducerError := newKafkaProducer(appConfig)