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/l10n/app_localizations_ja.dart

@@ -470,4 +470,66 @@ class AppLocalizationsJa extends AppLocalizations {
 
   @override
   String get processingProducts => '処理中...';
+
+  @override
+  String get clearAllProducts => 'すべてクリア';
+
+  @override
+  String get clearAllConfirmTitle => 'すべての商品をクリアしますか？';
+
+  @override
+  String get clearAllConfirmMessage => 'すべての商品が完全に削除されます。';
+
+  @override
+  String get addManually => '手動';
+
+  @override
+  String get scan => 'スキャン';
+
+  @override
+  String get addProduct => '追加';
+
+  @override
+  String get searchProducts => '製品を検索';
+
+  @override
+  String get searchProductsHint => '製品名を入力するか手動で追加してください';
+
+  @override
+  String noSearchResults(String query) {
+    return '「$query」の検索結果はありません';
+  }
+
+  @override
+  String get quantity => '数量';
+
+  @override
+  String get storageDays => '保存日数';
+
+  @override
+  String get addToShelf => 'パントリーに追加';
+
+  @override
+  String get errorGeneric => 'エラーが発生しました';
+
+  @override
+  String get nutritionOptional => '100gあたりの栄養素（任意）';
+
+  @override
+  String get calories => 'カロリー';
+
+  @override
+  String get protein => 'タンパク質';
+
+  @override
+  String get fat => '脂質';
+
+  @override
+  String get carbs => '炭水化物';
+
+  @override
+  String get fiber => '食物繊維';
+
+  @override
+  String get productAddedToShelf => 'パントリーに追加しました';
 }