Validate the request signature screenshot in pull requests

The screenshot generation added in #8090 should also be validated automatically in pull requests.

The goal is to detect when changes to the LibreSign interface make img/screenshot/request-signature.png outdated.

This issue depends on #8090.

What should be done

Add a GitHub Actions workflow that:

1. runs the screenshot generation command from #8090;
2. compares the generated image with the committed img/screenshot/request-signature.png;
3. uses pixelmatch or an equivalent small visual comparison library;
4. ignores insignificant rendering differences using a reasonable tolerance;
5. succeeds when the screenshots match within the allowed tolerance;
6. fails when a relevant visual difference is detected.

When the comparison fails, the workflow should:

• report the number or percentage of different pixels;
• generate a visual diff;
• upload the generated screenshot and diff as workflow artifacts;
• add or update a comment in the pull request.

Reference implementation

A similar implementation can be used as reference from LSTelegramNotify:

• [comparePngBuffers()](https://github.com/LibreCodeCoop/LSTelegramNotify/blob/c2f68cbdbe6318d134a5f2e79ccf27cded9106c0/scripts/screenshots/checker.mjs#L8-L47) shows how pixelmatch can compare two PNG buffers and report the number and percentage of different pixels;
• [assertScreenshotsUpToDate()](https://github.com/LibreCodeCoop/LSTelegramNotify/blob/c2f68cbdbe6318d134a5f2e79ccf27cded9106c0/scripts/screenshots/checker.mjs#L49-L124) shows how a failed comparison can generate a visual diff;
• [documentation-screenshots.yml](https://github.com/LibreCodeCoop/LSTelegramNotify/blob/c2f68cbdbe6318d134a5f2e79ccf27cded9106c0/.github/workflows/documentation-screenshots.yml) shows how screenshot validation and diff artifacts can be handled in GitHub Actions.

The LibreSign implementation does not need to copy this structure exactly.

Pull request comment

When a relevant difference is found, the workflow should leave a short comment explaining that the request signature screenshot may be outdated.

The comment should include:

• the detected difference;
• a note telling the contributor to run the screenshot generation command from #8090 if the visual change is expected;
• a reference to the workflow artifacts with the generated screenshot and visual diff.

The workflow should avoid creating duplicate comments. If a screenshot validation comment already exists, update it instead.

A hidden marker can be used to identify the comment, for example:

<!-- libresign-request-signature-screenshot-check -->

No comment is needed when the screenshot matches.

The workflow must not automatically commit the generated screenshot. Updating img/screenshot/request-signature.png should remain an explicit change made after reviewing the detected visual difference.

Acceptance criteria

• The screenshot generated by #8090 is compared with the committed img/screenshot/request-signature.png.
• pixelmatch or an equivalent approach is used for visual comparison.
• A documented tolerance prevents failures caused only by insignificant rendering differences.
• The workflow succeeds when the images match within the allowed tolerance.
• The workflow fails when a relevant visual difference is detected.
• The comparison reports the number or percentage of different pixels.
• A visual diff is generated when the comparison fails.
• The generated screenshot and visual diff are uploaded as GitHub Actions artifacts.
• A pull request comment explains when the screenshot may need to be updated.
• Repeated runs update the existing validation comment instead of creating duplicate comments.
• The workflow does not automatically commit screenshot changes.

Additional context

• If you have questions, feel free to ask in this issue.
• Give a ⭐️ star to this repository if you find LibreSign useful and would like to support the project.
• You can also join our community: https://t.me/LibreSign

Start with the screenshot generation command from #8090, the committed img/screenshot/request-signature.png, and the referenced checker.mjs and documentation-screenshots.yml examples. Build the GitHub Actions validation around the stated comparison, tolerance, diff artifacts, and pull request comment behavior; done means all acceptance criteria pass without automatically committing the screenshot.