Fix progress preview gallery selection

[JPPhoto]

Summary

Fixes image viewer progress preview behavior when selecting gallery images during generation. Previously, selecting a gallery image during an active progress preview cleared the preview immediately, leaving the selected image visible until the next progress update or completion.

When a progress preview is visible, selecting a gallery image now shows that selected image for 2 seconds before returning to the active progress preview. The preview continues updating behind that temporary reveal. The Info button now matches the visible state: it is enabled during the 2-second selected-image reveal and disabled again when the progress overlay returns.

Also fixes a race where a stale completion handoff could clear a newer generation's progress preview.

Related Issues / Discussions

QA Instructions

Manual behavior to verify:

1. Start a generation with Display Progress Images enabled.
2. While the progress preview is visible, select a different gallery image.
3. Confirm the selected image is shown for 2 seconds, then the progress preview returns.
4. Confirm new progress updates continue and the latest preview is shown after the reveal.
5. Confirm the Info button is enabled during the 2-second reveal and disabled while the progress overlay is visible.

Merge Plan

Checklist

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