EmbeddedChat 2026: Modernize APIs, Enable Matrix Federation, Add Pluggable AI, and Improve Mobile UX

EmbeddedChat is Rocket.Chat's embeddable chat widget, but RC 8.x broke several core endpoints, Matrix federation support is absent, there is no AI extensibility point, mobile responsiveness is inconsistent, and WCAG compliance is incomplete. This project restores RC 8.x compatibility and completes the DDP client migration, adds production-ready Matrix federation via a mode prop backed by an upstream RC core fix, introduces a standalone provider-agnostic AI adapter package, systematically fixes mobile viewport issues, and brings the widget to WCAG 2.1 AA compliance. Working POCs exist for all five goals