Feat(Canvas): Add button to hide preview stage thumbnails

[DustyShoe]

Summary

Adds a toggle to the canvas staging area toolbar to hide/show preview thumbnails while keeping the staging menu visible.

Behavior:

• Thumbnails can be collapsed and expanded with the new caret button.
• The visibility state persists across temporary mode switches/remounts.
• The state resets to visible when the staging session is cleared, such as after accept/commit or discard/reset.
• The thumbnail strip restore behavior was adjusted to avoid the visible jump/flicker when showing thumbnails again.

Related Issues / Discussions

QA Instructions

1. Generate images so the staging area appears on canvas.
2. Click the new Caret Down button and confirm thumbnails hide while the toolbar stays visible.
3. Click it again and confirm thumbnails return without the jump/flicker.
4. Switch tabs while preview is visible and confirm no jump/flicker on return to canvas tab.
5. Hide thumbnails, switch away from canvas mode, then return and confirm they are still hidden.
6. Accept/commit a staged image or clear/discard the staging session, then stage a new image and confirm thumbnails are visible again.

Merge Plan

Simple merge

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)