fix: align signer email contract with API runtime behavior

[vitormattos]

Summary

This PR aligns API contract, generated OpenAPI/types, and frontend runtime parsing for signer email in validation payloads.

The backend runtime can return signers without email for some identify-method flows, while contract/types previously required email.

Changes

• Update ResponseDefinitions signer summary contract to email?: ?string
• Regenerate OpenAPI specs (openapi.json, openapi-full.json)
• Regenerate OpenAPI TypeScript types (src/types/openapi/openapi.ts, src/types/openapi/openapi-full.ts)
• Update validation parser to accept nullable/omitted signer email
• Add/adjust parser tests for production-shaped payload with email: null and omitted email
• Update frontend types/components to safely handle nullable signer email

[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/7602/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 d3c3aaca 35280d6e 02fc19c5 1b11d491 4d091ff9 7548b4fa 318933c1 a7340f12 78210f5b 0bb4d0c7 e57535ff c33a04dc

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/7602/stable32

Error: Failed to push branch backport/7602/stable32: remote: Invalid username or token. Password authentication is not supported for Git operations.
fatal: Authentication failed for 'https://github.com/LibreSign/libresign.git/'

---

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

[vitormattos]

/backport to stable32