The `esign_url` claim adds a "Sign" action on PDFs/documents that embeds a self-hosted
DocuSeal (or any web e-sign tool), completing the BYO-embed set (terminal/PDF/office/e-sign).

- Mirrors office_url: esign_url may carry a {url} placeholder the UI substitutes with the
  selected file's presigned URL (URL-encoded, no injection). The operator's page builds the
  signing request → the core stays stateless.
- Claims::$esignUrl, http(s)-only (non-http dropped). Pure UI: sandboxed iframe + no new
  server endpoint → works in every mode.
- fm.js: esignUrl getter + canEsign() (pdf/doc/docx/odt + a file url) + openEsign() + a
  detail-panel button + a modal. i18n esign.title ×16. Forwarded by embed/node(+dist)/
  laravel/wordpress. docs/CONFIG.md. node 0.1.24.
- Test: esign_url forward + http/s-only; Playwright-verified button gating + {url} deep-link.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>