Add invite routes to backend and frontend; implement routing and UI components for invitation management

frontend/src/App.vue

@@ -77,6 +77,7 @@ const pageTitle = computed(() => {
     'settings-profile': 'Persönliches',
     'settings-password': 'Passwort ändern',
     'settings-timewish': 'Zeitwünsche',
+    'settings-invite': 'Einladen',
     'entries': 'Einträge',
     'stats': 'Statistiken'
   }