feat: add and enhance PermitsScreen and RequestPermitModal

- add LoadingScreen component
- update utilities

components/CalendarWidget.tsx

@@ -32,6 +32,7 @@ export default function CalendarWidget({ events, types }: CalendarWidgetProps) {
         return events.find(event => {
             // Logica semplice: controlla se la data cade nel range
             // Nota: per una logica perfetta sui range lunghi, servirebbe un controllo più approfondito
+            if (!event.start_date) return false;
             if (event.timeOffRequestType.name === 'Permesso') return event.start_date === dateStr;
             const end = event.end_date || event.start_date;
             return dateStr >= event.start_date && dateStr <= end;