feat(website): migrate to css modules and react 18

[ayuhito]

Mantine V7 has dropped Emotion in favour of CSS modules which is a far more performant and cleaner solution to use. Our biggest blocker for updating to React 18 was all the bugs associated with CSS-in-JS, so this change lets us use all the streaming goodies included in React 18 in future updates.

The migration to CSS modules should also include minor improvements to accessibility and design from our end (e.g. a completely rewritten combobox component).

Closes #768 (which was a hydration bug).

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Latest commit: ca20aa3

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[ayuhito]

Not sure why staging won't deploy from GH Actions whilst local deploy using the remote builder does pass. There shouldn't be any mixed context too considering the strict dockerignore. For now, I'll merge since prod is already running this branch.