feat(shopping): custom categories - add, rename, delete and reorder (#26)

- New DB table shopping_categories (migration v5) seeds 9 default
  categories with Lucide icons and sort_order
- Backend CRUD routes: GET/POST/PUT/DELETE /shopping/categories
  plus PATCH /shopping/categories/reorder
- Category validation now uses DB instead of hardcoded constant;
  items of deleted category are moved to the next available one
- Frontend shopping page loads categories from API, dropdown and
  grouping reflect custom order dynamically
- Settings -> Shopping section: list categories with up/down buttons,
  click-to-rename, delete with confirmation; add new categories inline
- i18n keys added in de/en/sv/it

public/locales/it.json

@@ -455,6 +455,19 @@
   "settings": {
     "title": "Impostazioni",
     "sectionDesign": "Aspetto",
+    "sectionShopping": "Spesa",
+    "shoppingCategoriesLabel": "Categorie spesa",
+    "shoppingCategoriesHint": "Aggiungi, rinomina, elimina o riordina le categorie.",
+    "shoppingCategoryPlaceholder": "Nuova categoria…",
+    "shoppingCategoryRenameHint": "Clicca per rinominare",
+    "shoppingCategoryRenamePrompt": "Nuovo nome categoria:",
+    "shoppingCategoryMoveUp": "Sposta categoria su",
+    "shoppingCategoryMoveDown": "Sposta categoria giu",
+    "shoppingCategoryDelete": "Elimina categoria",
+    "shoppingCategoryDeleteConfirm": "Eliminare la categoria \"{{name}}\"? Gli articoli esistenti verranno spostati alla categoria successiva.",
+    "shoppingCategoryAdded": "Categoria aggiunta.",
+    "shoppingCategoryRenamed": "Categoria rinominata.",
+    "shoppingCategoryDeleted": "Categoria eliminata.",
     "sectionAccount": "Il mio account",
     "sectionCalendarSync": "Sincronizzazione calendario",
     "sectionFamily": "Membri della famiglia",