Fix layouting issue on welcome screen for narrow window, triggering debug assertion

[Wumpf]

What

• Fixes Layout code panics #5649

This is a speculative fix, didn't have a repro. From the callstack we know that a certain ui.set_max_width(column_width - 8.0); might have gotten a too small width. Looking at how column_width is produced this may very well happen if for some reason there's very little available_width.

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  • 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
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[Wumpf]

correction! I got a repro!
Without this fix you can crash (on debug) the viewer by expanding the left (empty) blueprint panel such that there's no space for example columns anymore

[github-actions]

Size changes

Name	main	5650/merge	Change
rrt-star.rrd	10.46 MiB	1.80 MiB	-82.79%