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/sv.json

@@ -455,6 +455,19 @@
   "settings": {
     "title": "Inställningar",
     "sectionDesign": "Utseende",
+    "sectionShopping": "Inköp",
+    "shoppingCategoriesLabel": "Inköpskategorier",
+    "shoppingCategoriesHint": "Lägg till, byt namn, ta bort eller sortera om kategorier.",
+    "shoppingCategoryPlaceholder": "Ny kategori…",
+    "shoppingCategoryRenameHint": "Klicka för att byta namn",
+    "shoppingCategoryRenamePrompt": "Nytt kategorinamn:",
+    "shoppingCategoryMoveUp": "Flytta kategori uppåt",
+    "shoppingCategoryMoveDown": "Flytta kategori nedåt",
+    "shoppingCategoryDelete": "Ta bort kategori",
+    "shoppingCategoryDeleteConfirm": "Ta bort kategorin \"{{name}}\"? Befintliga artiklar flyttas till nästa kategori.",
+    "shoppingCategoryAdded": "Kategori tillagd.",
+    "shoppingCategoryRenamed": "Kategori omdöpt.",
+    "shoppingCategoryDeleted": "Kategori borttagen.",
     "sectionAccount": "Mitt konto",
     "sectionCalendarSync": "Kalendersynkronisering",
     "sectionFamily": "Familjemedlemmar",