feat(ui): prompt expansion

[maryhipp]

Summary

• Adds features for prompt expansion, either string -> string or image -> string. The latter can be done by drag-n-dropping or right-clicking an image in the gallery, or by using the Prompt Expansion context menu to upload an image that is then analyzed.
• Once a result comes back, user needs to accept or discard it.
• During prompt expansion, prompt features are locked and the user cannot queue any further items until that flow is complete so as to avoid enqueueing with unexpected prompt content.

Related Issues / Discussions

QA Instructions

Merge Plan

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)

[psychedelicious]

I've submitted #8146 which allows the execution-tracking logic in this PR to be greatly simplified.