Components.AI accessibility and adaptive presentation

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

The built-in UI lacks localization, a coherent icon strategy, streaming announcements, reduced-motion handling, forced-colors support, dark-mode defaults, and RTL support.

### Describe the solution you'd like

Track independently deliverable cross-cutting accessibility and adaptive-presentation improvements, using the same accessibility bar as other shipped ASP.NET Core components.

This is a workstream under #68227. Its independently mergeable tasks are represented as native sub-issues.

### Additional context

Covers A4-A5, H8, and I1-I9 from #68227. Approval-specific accessibility remains related to the approval/tool-call experience.

Read the parent issue #68227 and the native sub-issues representing the independently mergeable tasks. Start with a specific sub-issue covering one of A4-A5, H8, or I1-I9; completion should meet that task's acceptance criteria and the accessibility bar used by other shipped ASP.NET Core components.

Written by the indexing model from the issue text.