Extracts PDF text during upload via Pdfium and stores it in attachment
metadata (pdf.pages_text), then surfaces it in visually-hidden sr-only
regions in both the signing form and submission preview views. Also adds
alt text to template builder page images and ARIA role/label to the
page-container custom element.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>