Improve Responsive Design accessibility

[issackjohn]

Improve the Responsive Design benchmark's accessibility while keeping its existing layout and workload intact.

• Increase contrast for navigation, chat, newsletter, and recipe controls.
• Add visible keyboard focus styles to interactive chat controls.
• Replace the ambiguous overflow icon with an accessible disclosure of hidden ribbon items.
• Close stale overflow content after breakpoint changes and preserve focus when controls move into or out of overflow.

Validation

• Responsive Design production build
• Chrome focus-restoration smoke test
• Independent code review with no remaining findings

Performance

Crossbench Speedometer 3 Responsive-Design, 10 runs x 10 iterations:

Browser	Base	Patch	Delta	Bootstrap 95% CI
Chrome 152	132.492 ms	132.516 ms	+0.019%	-0.155% to +0.195%
Edge 151	110.157 ms	112.079 ms	+1.744%	-1.103% to +4.745%

Both intervals include zero, so neither browser shows a measurable regression. All Chrome and Edge runs completed without failures.

Safari 26.6 could not produce a final comparison because SafariDriver exited during both the patched run and an immediate unchanged-base smoke run, indicating an automation-environment failure rather than a code-specific result.

This fork-only stacked PR targets track-carousel-resize-events, so main-only CI does not run here.