Open WebUI vulnerable to Stored XSS via iFrame in citations model

Open WebUI is a self-hosted artificial intelligence platform designed to operate entirely offline. Prior to version 0.7.0, aanually modifying chat history allows setting the `html` property within document metadata. This causes the frontend to enter a code path that treats document contents as HTML, and render them in an iFrame when the citation is previewed. This allows stored XSS via a weaponized document payload in a chat. The payload also executes when the citation is viewed on a shared chat. Version 0.7.0 fixes the issue.