[Feature]: `Update snapshot` sticky header in UI Mode Attachments panel

🚀 Feature Request

Add an Update snapshot action to each IMAGE DIFF section in UI Mode's Attachments panel.

The existing IMAGE DIFF header would remain sticky while scrolling through the comparison, with Update snapshot replacing that screenshot's expected snapshot with the actual screenshot.

This follows existing UI Mode patterns where actions are placed in a sticky header for the content they operate on, for example the Source panel actions:

https://github.com/user-attachments/assets/8d2c998b-2e56-4132-bccc-d3b23305bb1c

Example

After a failed test run with failing snapshots:

1. Review a failed screenshot using the existing Diff / Actual / Expected views
2. Click Update snapshot for any intentional change which should have the current snapshot replaced by the Actual snapshot
3. Continue reviewing the remaining image diffs individually

The existing Attachments panel:

The proposed action would be added to the existing IMAGE DIFF header:

IMAGE DIFF                                         Update snapshot
──────────────────────────────────────────────────────────────────
           Diff  Actual  Expected  Side by side  Slider

The header would remain sticky while scrolling through a large comparison. The action only appears for IMAGE DIFF sections, not other attachments.

Alternative designs can be seen in prior art:

• https://github.com/abhivaikar/pw-ui-review

Motivation

When several screenshot assertions fail, UI Mode lets you review each diff, but not update an intentional change directly from that diff.

I'm aware that Playwright v1.56+ supports updateSnapshots in UI Mode (issue #37007 + PR #37098), but the current workflow has a few drawbacks:

1. If users enable it before running, snapshots are updated before users know whether the changes are correct
2. If users run tests normally to failure and review first, accepting a change requires enabling it and running the test again, which can be slow, flaky, and interrupts the review flow
3. Because updating is run-level, other changed snapshots can be updated before users have reviewed them