avoid `await` at top-level in setup - fixes frontend tests

What does this PR do?

Putting an await at the top level inside a Vue setup script marks the whole setup as async (correctly), which confuses the tests.

We don't really need to block setup until we've talked with listenbrainz: we can fire the request, and update listenbrainzdata once we have the response. Of course we also need to make listenbrainzdata a ref, otherwise the UI won't update.

Tested, it works.

After merging this and the future-2024-05-31 branch, we should be able to run tests in our CI pipeline!

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