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

frontend/src/router/index.js

@@ -15,6 +15,7 @@ import Vacation from '../views/Vacation.vue'
 import Sick from '../views/Sick.vue'
 import Workdays from '../views/Workdays.vue'
 import Calendar from '../views/Calendar.vue'
+import Holidays from '../views/Holidays.vue'
 
 const router = createRouter({
   history: createWebHistory(import.meta.env.BASE_URL),
@@ -91,6 +92,12 @@ const router = createRouter({
       component: Calendar,
       meta: { requiresAuth: true }
     },
+    {
+      path: '/admin/holidays',
+      name: 'admin-holidays',
+      component: Holidays,
+      meta: { requiresAuth: true, requiresAdmin: true }
+    },
     {
       path: '/entries',
       name: 'entries',