Accessibility: aria-live for streamed answers, contrast on citation chips

[ronishgeorge]

Lighthouse audit flagged two issues on the Ask interface:

1. The answer region lacks aria-live="polite" — screen readers don't announce content as it streams in.
2. The citation-chip color combo (signal-300 on bone-50) is close to but not quite WCAG-AA on small text.

• Add aria-live to the answer container
• Bump citation-chip text to signal-600, ensure 4.5:1 contrast on bone-50 background
• Verify with axe-core in CI