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/de.json

@@ -226,6 +226,8 @@
     "addIngredient": "Zutat hinzufügen",
     "ingredientNamePlaceholder": "Zutat",
     "ingredientQtyPlaceholder": "Menge",
+    "ingredientCategoryLabel": "Kategorie",
+    "ingredientCategoryDefault": "Sonstiges",
     "removeIngredient": "Zutat entfernen",
     "transferLabel": "Zutaten auf Einkaufsliste übertragen",
     "transferNow": "Jetzt übertragen",