Fix ugly UI for some arrow data

[emilk]

What

Improve the display of arbitrary arrow data, slightly.

There were several problems:

• We were displaying huge datatypes (unions)
• We were not respecting the max toolbar size
• We were falling back on datatype too early

Before:

After:

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• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

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