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.dart

@@ -24,6 +24,8 @@ class CatalogProduct {
   final double? fatPer100g;
   @JsonKey(name: 'carbs_per_100g')
   final double? carbsPer100g;
+  @JsonKey(name: 'fiber_per_100g')
+  final double? fiberPer100g;
 
   const CatalogProduct({
     required this.id,
@@ -37,6 +39,7 @@ class CatalogProduct {
     this.proteinPer100g,
     this.fatPer100g,
     this.carbsPer100g,
+    this.fiberPer100g,
   });
 
   /// Display name is the server-resolved canonical name (language-aware from backend).