Move from headless-ui to melt-ui

[MrGreenTea]

We've used the svelte-headlessui package for some components. This was before we found melt-ui.

There are multiple reasons to remove svelte-headlessui:

• svelte-headlessui is not updated anymore
• svelte-headlessui causes some warnings
• melt-ui is more idiomatic for svelte

[MrGreenTea]

Components that still use headless-ui:

• website/src/components/support-us/Disclosure.svelte

  Line 3 in 029b6f5

  import { Disclosure, DisclosureButton, DisclosurePanel } from '@rgossiaux/svelte-headlessui';

• website/src/components/support-us/DonationMethodTabs.svelte

  Line 3 in 029b6f5

  import { Tab, TabGroup, TabList, TabPanel, TabPanels } from '@rgossiaux/svelte-headlessui';

[MrGreenTea]

One more reason: it does not work with svelte 5: #441