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

server/db.js

@@ -331,6 +331,30 @@ const MIGRATIONS = [
       CREATE INDEX IF NOT EXISTS idx_tasks_due            ON tasks(due_date);
     `,
   },
+  {
+    version: 5,
+    description: 'Einkaufskategorien als eigene Tabelle (anpassbar, sortierbar)',
+    up: `
+      CREATE TABLE IF NOT EXISTS shopping_categories (
+        id         INTEGER PRIMARY KEY AUTOINCREMENT,
+        name       TEXT    NOT NULL UNIQUE,
+        icon       TEXT    NOT NULL DEFAULT 'tag',
+        sort_order INTEGER NOT NULL DEFAULT 0,
+        created_at TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))
+      );
+
+      INSERT INTO shopping_categories (name, icon, sort_order) VALUES
+        ('Obst & Gemüse',   'apple',           0),
+        ('Backwaren',        'wheat',           1),
+        ('Milchprodukte',    'milk',            2),
+        ('Fleisch & Fisch',  'beef',            3),
+        ('Tiefkühl',         'snowflake',       4),
+        ('Getränke',         'cup-soda',        5),
+        ('Haushalt',         'spray-can',       6),
+        ('Drogerie',         'pill',            7),
+        ('Sonstiges',        'shopping-basket', 8);
+    `,
+  },
 ];
 
 /**