fix(website): nav contrast, accessibility polish, and visual improvements for Astro site

[ashleyshaw]

Summary

Follow-up accessibility and visual polish for the Awesome GitHub Astro site (built in #840). Addresses nav header contrast, colour token consistency, skip-link focus management, and spacing issues discovered during review.

Scope

• Nav header: raise background opacity so light-mode links meet WCAG AA contrast
• Skip-to-content link: wired to <main id="main-content"> with tabindex="-1" for programmatic focus
• SearchPalette: focus restoration on close, Tab focus trapping (WCAG 2.1.2, 2.4.3)
• Colour tokens: replace var(--c-light-blue) with var(--accent) on tag/badge elements to fix contrast in light mode
• Mobile gap: remove stale margin-top: 72px from 9+ pages (leftover from fixed → sticky nav transition)
• Trailing slashes: fix category and CTA hrefs in index.astro for GitHub Pages static routing
• Broken links: update 5 references to removed sub-routes (/getting-started/implementation-roadmap/ → /getting-started/)
• localStorage guards: wrap branch read/write in try/catch for private browsing

Related

Closes #840 (follow-up polish to the Astro rebuild)

[linear-code]

LS-940

LS-1083

[github-actions]

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[github-actions]

Metadata governance

• Item: fix(website): nav contrast, accessibility polish, and visual improvements for Astro site #842
• Assignee: ashleyshaw
• Milestone: fix(website): nav contrast, accessibility polish, and visual improvements for As
• Linked issues/PRs: feat: Rebuild Awesome GitHub site — complete Astro conversion #840