fix(mantine): fix FOUC and handle error styles

[markdalgleish]

This fixes a couple of issues with the Mantine example:

• There was a flash of unstyled content on load due to missing <StylesPlaceholder />.
• The styles were removed from the document when navigating between error routes and non-error routes. This is due to the way in which Remix re-renders the entire document from the root when rendering a root-level error component. The fix is based on correiarmjoao/remix-with-mantine by @correiarmjoao (shared in [Bug]: Style tags disappear from Head during Error/Catch Boundary remix#1136).

[MichaelDeBoey]

Not sure if e should merge all of these changes tbh, as we try to have our examples as small as possible and this seems to add a lot of unnecessary boilerplate to the example

I think the goal here is: get Mantine working + have 1 page of styling
WDYT?

[markdalgleish]

I was matching a similar workaround in the Emotion example.

I definitely understand the concern, but I also feel like this repo doubles as a set of manual tests for the different integrations. In this case we're working around a particular issue quite heavily and I think it's helpful that the example allows us to exercise that code path.