fix: handle visible signatures for envelope child files

[vitormattos]

Summary

• detect the current signer across envelope child files when visible signatures are required
• apply the same rule in the signer view and signing requirement validation
• add unit, component, and Playwright regression coverage for issue unauthenticated user cannot sign when multiple PDFs are in the sign request #7344, with a readable end-to-end scenario

Testing

• npm run ts:check
• npm run test
• docker compose exec -u www-data -w /var/www/html/apps-extra/libresign playwright npx playwright test playwright/e2e/sign-envelope-unauthenticated-visible-signature.spec.ts --output=/tmp/playwright-issue-7344-readable

Fixes #7344

[vitormattos]

/backport to stable33

[vitormattos]

/backport to stable32

[backportbot-libresign]

The backport to stable32 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable32
git pull origin stable32

# Create the new backport branch
git checkout -b backport/7383/stable32

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick fcb4aa9f 372b852a 57ef6b81 9e855f7f 007076c3 472a4e2f e50304ac 670104f3 0182377a 011a322a

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/7383/stable32

Error: Failed to push branch backport/7383/stable32: remote: Invalid username or token. Password authentication is not supported for Git operations.
fatal: Authentication failed for 'https://github.com/LibreSign/libresign.git/'

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[vitormattos]

/backport to stable32