Improve welcome screen for small screens

[emilk]

Makes the welcome screen use more of the available space.

Important for small screens, mobile, or embedded web viewers.

What

• Hide blueprint panel
• Reduce vertical space
• Fix title casing

Before (0.16 viewer)

After

Mobile

Checklist

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

• PR Build Summary
• Recent benchmark results
• Wasm size tracking

To run all checks from main, comment on the PR with @rerun-bot full-check.