feat(ui): iterate on combobox component, model picker

[psychedelicious]

Summary

• Iterate on combobox/picker component (layout, functionality, perf, flexibility)
• Port over a good chunk of model dropdowns to it
• Start moving non-model drop-downs to native select - still a fair few to port
• Need to get the thing to auto-size correctly

Related Issues / Discussions

n/a

QA Instructions

n/a

Merge Plan

n/a

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)