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/shared/models/product.g.dart

@@ -19,6 +19,7 @@ CatalogProduct _$CatalogProductFromJson(Map<String, dynamic> json) =>
       proteinPer100g: (json['protein_per_100g'] as num?)?.toDouble(),
       fatPer100g: (json['fat_per_100g'] as num?)?.toDouble(),
       carbsPer100g: (json['carbs_per_100g'] as num?)?.toDouble(),
+      fiberPer100g: (json['fiber_per_100g'] as num?)?.toDouble(),
     );
 
 Map<String, dynamic> _$CatalogProductToJson(CatalogProduct instance) =>
@@ -34,4 +35,5 @@ Map<String, dynamic> _$CatalogProductToJson(CatalogProduct instance) =>
       'protein_per_100g': instance.proteinPer100g,
       'fat_per_100g': instance.fatPer100g,
       'carbs_per_100g': instance.carbsPer100g,
+      'fiber_per_100g': instance.fiberPer100g,
     };