Refactor date handling components: Replace DateRangeDropdown with DateRangeCollapse

- Introduced DateRangeCollapse.svelte to manage date range selection with timezone support.
- Removed DateRangeDropdown.svelte as it was redundant.
- Updated LodgingModal and TransportationModal to utilize DateRangeCollapse for date selection.
- Enhanced date conversion utilities to handle all-day events correctly.
- Adjusted TimezoneSelector for improved accessibility and focus management.
- Updated date handling logic in dateUtils.ts to support all-day events.
- Modified test page to reflect changes in date range component usage.

frontend/src/routes/datetest/+page.svelte

@@ -1,13 +0,0 @@
-<script>
-	import DateRangeDropdown from '$lib/components/DateRangeDropdown.svelte';
-
-	let utcStartDate = '';
-	let utcEndDate = '';
-</script>
-
-<DateRangeDropdown bind:utcStartDate bind:utcEndDate />
-
-<p>{new Date(utcStartDate).toLocaleString()} - {new Date(utcEndDate).toLocaleString()}</p>
-
-<p>UTC Start Date: {utcStartDate}</p>
-<p>UTC End Date: {utcEndDate}</p>