feat: enhance absence management with calendar integration and error handling

- Added functionality to check for date overlaps with existing absence requests in the absence calendar.
- Implemented a modal to display error messages when users attempt to create overlapping absence requests.
- Updated the calendar component to visually indicate blocked days due to existing approved or pending absence requests.
- Improved user feedback by providing alerts for unavailable periods and enhancing the overall user experience in absence management.

apps/web/src/lib/components/ErrorModal.svelte

@@ -39,7 +39,7 @@
       <p class="py-4 text-base-content">{message}</p>
       {#if details}
         <div class="bg-base-200 rounded-lg p-3 mb-4">
-          <p class="text-sm text-base-content/70 font-mono">{details}</p>
+          <p class="text-sm text-base-content/70 whitespace-pre-line">{details}</p>
         </div>
       {/if}
       <div class="modal-action">