fix(webapp): enforce accessible image and role semantics (#4693)

## Summary

Enable foundational JSX accessibility checks for image text alternatives
and valid ARIA roles.

The avatar color picker now has an explicit accessible name and
decorative image alternative. Dashboard chat styling props no longer
reuse the reserved DOM `role` name.

Base: [#4692](https://github.com/triggerdotdev/trigger.dev/pull/4692)