Show the root entity "/" in the streams panel

[emilk]

What

• Closes https://github.com/rerun-io/rerun/issues/8138

Checklist

• [x] I have read and agree to Contributor Guide and the Code of Conduct

• [x] I've included a screenshot or gif (if applicable)

• [x] I have tested the web demo (if applicable):

  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer

• [x] The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

• [x] If applicable, add a new check to the release checklist!

• [x] If have noted any breaking changes to the log API in CHANGELOG.md and the migration guide

• PR Build Summary

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