Show HN: Extend UI – open-source UI kit for modern document apps

We're open-sourcing Extend UI, a customizable UI kit for modern document apps, with 14 components and examples for PDF, DOCX, and XLSX viewers. It's MIT licensed and useful for building document processing agents or internal tooling. The kit was initially built for Extend.ai but has been improved to handle millions of pages per day. It's available for community support and improvement. Users can find a demo video at https://share.extend.ai/kRmSGKRF.