🐛 `startDate` overrides `defaultYear` / `defaultMonth`

[oscarhermoso]

Describe the bug

When both startDate and defaultYear / defaultMonth are defined, then defaultYear / defaultMonth is ignored.

I understand that this might be expected behaviour, but currently, there is no way to specify a startDate and also set the defaultMonth / defaultYear to a different value.

To Reproduce
Steps to reproduce the behavior:

1. Set up a date picker with below code:

<!-- src/routes/book-appointment/[[date]]+page.svelte -->

<script lang="ts">
	import { page } from '$app/stores';

	let selectedDate = $page.params.date ? new Date($page.params.date) : undefined;
	let defaultDate = $page.url.searchParams.has('viewFrom') ? new Date($page.params.get('viewFrom')) : undefined;
</script>

<DatePicker
	bind:startDate={selectedDate}
	showYearControls={false}
	defaultYear={defaultDate?.getFullYear()}
	defaultMonth={defaultDate?.getMonth()}
	isOpen={true}
	alwaysShow={true}
	enableFutureDates={true}
	enablePastDates={false}
/>

2. Go to URL http://localhost:5173/book-appointment/2024-03-15?viewFrom=2025-06-01

Expected behavior

I expect the calendar to show the month of June, and if I navigate back to June, then the 15th of March will be selected.

Actual behavior

The calendar shows the month of March, with the 15th of March selected.