feat(dashboard): add shopping list widget

Show shopping lists with open items directly on the dashboard.
Each list displays a progress bar, the first few unchecked items,
and a "+N more" overflow indicator. Widget only appears when there
are lists with open items.

Backend: new shoppingLists query in /api/v1/dashboard (up to 3 lists,
6 open items each). Frontend: renderShoppingLists() widget following
existing widget pattern. CSS: compact list/progress/item styles.
i18n: shoppingMore key added to de/en/it.

Requested in discussion #9

public/locales/en.json

@@ -75,7 +75,8 @@
     "overdue": "Overdue",
     "dueSoon": "Due today",
     "dueTomorrow": "Due tomorrow",
-    "allDay": "All day"
+    "allDay": "All day",
+    "shoppingMore": "+{{count}} more"
   },
 
   "tasks": {