Components.AI: Add jump-to-latest and unread indication

### Is your feature request related to a problem? Please describe the problem.

Readers who scroll away from the end have no indication that new content arrived and no direct way to return.

### Describe the solution you'd like

Add a jump-to-latest affordance with an unread/new-message indicator, integrated with configured follow behavior and keyboard/accessibility semantics.

### Additional context

Leaf task for #68227 requirement B3. Builds on the virtualized message list.

Start with requirement B3 in #68227 and trace the virtualized message list mentioned here. Identify where follow behavior is configured and how keyboard and accessibility semantics are handled. Done means readers away from the end can see an unread/new-message indication and use a jump-to-latest affordance that respects configured follow behavior and accessibility expectations.

Written by the indexing model from the issue text.