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/de.json

@@ -455,6 +455,19 @@
   "settings": {
     "title": "Einstellungen",
     "sectionDesign": "Design",
+    "sectionShopping": "Einkauf",
+    "shoppingCategoriesLabel": "Einkaufskategorien",
+    "shoppingCategoriesHint": "Kategorien hinzufügen, umbenennen, löschen oder sortieren.",
+    "shoppingCategoryPlaceholder": "Neue Kategorie…",
+    "shoppingCategoryRenameHint": "Klicken zum Umbenennen",
+    "shoppingCategoryRenamePrompt": "Neuer Kategoriename:",
+    "shoppingCategoryMoveUp": "Kategorie nach oben",
+    "shoppingCategoryMoveDown": "Kategorie nach unten",
+    "shoppingCategoryDelete": "Kategorie löschen",
+    "shoppingCategoryDeleteConfirm": "Kategorie \"{{name}}\" löschen? Vorhandene Artikel werden der nächsten Kategorie zugeordnet.",
+    "shoppingCategoryAdded": "Kategorie hinzugefügt.",
+    "shoppingCategoryRenamed": "Kategorie umbenannt.",
+    "shoppingCategoryDeleted": "Kategorie gelöscht.",
     "sectionAccount": "Mein Konto",
     "sectionCalendarSync": "Kalender-Synchronisation",
     "sectionFamily": "Familienmitglieder",