Add holidays routes to backend and frontend; implement holiday associations and update UI components for admin holidays management

frontend/src/App.vue

@@ -72,6 +72,7 @@ const pageTitle = computed(() => {
     'sick': 'Krankheit',
     'workdays': 'Arbeitstage',
     'calendar': 'Kalender',
+    'admin-holidays': 'Feiertage',
     'entries': 'Einträge',
     'stats': 'Statistiken'
   }