feat: per-ingredient category selection for shopping list transfer (closes #33)

When adding ingredients in the meal editor, each ingredient now has a
category dropdown. Categories are stored on the ingredient and applied
automatically when transferring to the shopping list, so items appear
pre-grouped by category without manual re-sorting.

public/locales/ja.json

@@ -226,6 +226,8 @@
     "addIngredient": "材料を追加",
     "ingredientNamePlaceholder": "材料",
     "ingredientQtyPlaceholder": "量",
+    "ingredientCategoryLabel": "カテゴリ",
+    "ingredientCategoryDefault": "その他",
     "removeIngredient": "材料を削除",
     "transferLabel": "材料を買い物リストに追加",
     "transferNow": "今すぐ追加",
@@ -596,4 +598,4 @@
     "unitMonth": "ヶ月",
     "unitMonths": "ヶ月"
   }
-}
+}