ci: improve README screenshot failure report

[vitormattos]

This improves the README screenshot failure feedback in pull requests.

Changes:

• include structured comparison metadata in the screenshot artifact;
• show difference percentage, allowed tolerance and changed pixel count at the top of the comment;
• keep the visual diff expanded and move expected/generated screenshots into collapsible sections;
• link directly to the failed workflow run;
• update one persistent PR comment instead of creating a new comment on every failure;
• publish the same report to the GitHub Actions job summary.

The report still uses the existing trusted workflow_run flow and does not execute pull request code with elevated permissions.

[github-actions]

README screenshot check failed

The generated README screenshot differs from the committed version.

Expected

Generated

Diff

If the visual change is expected, run npm run screenshots:update and commit the updated screenshot.