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_ja.arb

@@ -172,5 +172,32 @@
   "jobTypeReceipt": "レシート",
   "jobTypeProducts": "商品",
   "scanSubmitting": "送信中...",
-  "processingProducts": "処理中..."
+  "processingProducts": "処理中...",
+  "clearAllProducts": "すべてクリア",
+  "clearAllConfirmTitle": "すべての商品をクリアしますか？",
+  "clearAllConfirmMessage": "すべての商品が完全に削除されます。",
+  "addManually": "手動",
+  "scan": "スキャン",
+  "addProduct": "追加",
+  "searchProducts": "製品を検索",
+  "searchProductsHint": "製品名を入力するか手動で追加してください",
+  "noSearchResults": "「{query}」の検索結果はありません",
+  "@noSearchResults": {
+    "placeholders": {
+      "query": {
+        "type": "String"
+      }
+    }
+  },
+  "quantity": "数量",
+  "storageDays": "保存日数",
+  "addToShelf": "パントリーに追加",
+  "errorGeneric": "エラーが発生しました",
+  "nutritionOptional": "100gあたりの栄養素（任意）",
+  "calories": "カロリー",
+  "protein": "タンパク質",
+  "fat": "脂質",
+  "carbs": "炭水化物",
+  "fiber": "食物繊維",
+  "productAddedToShelf": "パントリーに追加しました"
 }