Remediate homepage hero and search color-contrast (WCAG AA)

[WilliamBerryiii]

The Docusaurus documentation site homepage flagged several WCAG 2.x color-contrast items in Accessibility Insights (hero title/subtitle, secondary CTA, hero background pattern, and the navbar search input).

Scope

• Pin navbar search-input text/placeholder/background/border colors for light and dark themes to a deterministic >=4.5:1.
• Increase the hero gradient/pattern contrast margin so automated tools stop flagging white-on-blue text.
• Add automated coverage so contrast regressions are caught in CI.

Acceptance criteria

• Homepage hero and navbar search pass an axe-core scan in light and dark themes.
• Dark-theme documentation page passes an axe-core scan (in-paragraph link contrast).
• Tests run as part of the existing Docusaurus e2e suite.

Surfaced via Accessibility Insights manual review of the published site.