AI-native chat components

[glebfox]

The stock Vaadin components were built for human-to-human chat, not for talking to an AI. Avatars,
sender names, and timestamps are first-class; the affordances of a modern AI chat are not. This
mismatch is the core reason to build our own.

[glebfox]

For QA: check doc for the detailed info - jmix-framework/jmix-docs#188