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/internal/domain/userproduct/repository.go

@@ -106,6 +106,16 @@ func (r *Repository) Update(requestContext context.Context, id, userID string, r
 	return userProduct, scanError
 }
 
+// DeleteAll removes all user products for the given user.
+func (r *Repository) DeleteAll(requestContext context.Context, userID string) error {
+	_, execError := r.pool.Exec(requestContext,
+		`DELETE FROM user_products WHERE user_id = $1`, userID)
+	if execError != nil {
+		return fmt.Errorf("delete all user products: %w", execError)
+	}
+	return nil
+}
+
 // Delete removes a user product. Returns ErrNotFound if it does not exist or belongs to a different user.
 func (r *Repository) Delete(requestContext context.Context, id, userID string) error {
 	tag, execError := r.pool.Exec(requestContext,