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/core/router/app_router.dart

@@ -13,6 +13,7 @@ import '../../features/profile/profile_provider.dart';
 import '../../shared/models/user.dart';
 import '../../features/products/products_screen.dart';
 import '../../features/products/add_product_screen.dart';
+import '../../features/products/product_search_screen.dart';
 import '../../features/products/product_job_history_screen.dart';
 import '../../features/scan/product_job_watch_screen.dart';
 import '../../features/scan/scan_screen.dart';
@@ -107,6 +108,11 @@ final routerProvider = Provider<GoRouter>((ref) {
         path: '/products/add',
         builder: (_, __) => const AddProductScreen(),
       ),
+      // Product search — search catalog before falling back to manual add.
+      GoRoute(
+        path: '/products/search',
+        builder: (_, __) => const ProductSearchScreen(),
+      ),
       // Shopping list — full-screen, no bottom nav.
       GoRoute(
         path: '/menu/shopping-list',