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/en.json

@@ -455,6 +455,19 @@
   "settings": {
     "title": "Settings",
     "sectionDesign": "Appearance",
+    "sectionShopping": "Shopping",
+    "shoppingCategoriesLabel": "Shopping Categories",
+    "shoppingCategoriesHint": "Add, rename, delete or reorder categories.",
+    "shoppingCategoryPlaceholder": "New category…",
+    "shoppingCategoryRenameHint": "Click to rename",
+    "shoppingCategoryRenamePrompt": "New category name:",
+    "shoppingCategoryMoveUp": "Move category up",
+    "shoppingCategoryMoveDown": "Move category down",
+    "shoppingCategoryDelete": "Delete category",
+    "shoppingCategoryDeleteConfirm": "Delete category \"{{name}}\"? Existing items will be moved to the next category.",
+    "shoppingCategoryAdded": "Category added.",
+    "shoppingCategoryRenamed": "Category renamed.",
+    "shoppingCategoryDeleted": "Category deleted.",
     "sectionAccount": "My Account",
     "sectionCalendarSync": "Calendar Sync",
     "sectionFamily": "Family Members",