[AnnouncementBar] Add variants, dismissible support, and fix typo #596

Summary

Improvements to the AnnouncementBar component to fix accessibility issues, add missing features, and improve consistency.

Improvements

2. Add dismissible / onDismiss support

No way to dismiss/close the bar. A common pattern is an onDismiss callback with a close button.

3. Accept ReactNode for text prop

The text prop only accepts string. It could accept ReactNode for richer content (e.g., links, bold text).

4. Add warning, success, info variants

Only 3 variants exist (normal, gradient, error). Common additions: warning, success, info.

6. Fix announementBar typo

The CVA variable name announementBar is missing a c — should be announcementBar.

Moved to Cross-Cutting Issues

The following items have been moved to cross-cutting issues that address them across all components:

• Replace hardcoded CSS values with design tokens (Replace hardcoded CSS values with design tokens #672)
• Accessibility baseline (Accessibility baseline: aria attributes, roles, and semantic HTML #673)