Add default category icon and improve visit display:

- Set default icon for empty category in AdventureModal
- Enhance layout for visit buttons and validation messages in DateRangeCollapse
- Update localization files to include "no visits" strings in multiple languages

frontend/src/locales/no.json

@@ -304,7 +304,8 @@
     "no_ordered_items": "Legg til varer med datoer i samlingen for å se dem her.",
     "ordered_itinerary": "Bestilt reiserute",
     "sunrise_sunset": "Soloppgang",
-    "timezone": "Tidssone"
+    "timezone": "Tidssone",
+    "no_visits": "Ingen besøk"
   },
   "worldtravel": {
     "country_list": "Liste over land",