feat: improve accessibility — keyboard nav, focus states, ARIA (Closes #35)

[laurentketterle-hub]

Summary

Adds comprehensive accessibility enhancements for keyboard navigation, focus states, and ARIA support — meeting WCAG 2.1 AA criteria.

What's Included

public/assets/css/accessibility.css

• Skip Navigation Link: Hidden link becomes visible on focus, jumps to main content
• Focus Indicators: :focus-visible outlines on all interactive controls with accent glow
• Sidebar Navigation: Inline focus styles for nav links
• Modal/Panel Focus Trap: Visible focus ring inside dialogs with aria-modal="true"
• Reduced Motion: @media (prefers-reduced-motion) disables all animations
• High Contrast Mode: @media (forced-colors: active) for Windows High Contrast
• Screen Reader Only: .sr-only utility for hidden descriptive text
• Live Region Announcer: Polite aria-live region for dynamic content updates
• Form Validation: aria-invalid styling with red borders and alert roles
• Disclosure Widgets: aria-expanded chevron indicators

public/assets/js/accessibility.js

• Skip Link Injection: Auto-injects skip-to-content link
• Live Region Announcer: Singleton aria-live="polite" element with debounced announcements
• Focus Trap: Keyboard trap for modals/dialogs (Tab/Shift+Tab cycling)
• Modal Observer: MutationObserver auto-detects new [aria-modal="true"] elements
• Card Keyboard Nav: Enter/Space activates card-like [role="button"] elements
• Global API: window.__a11y.announce() and window.__a11y.trapFocus() for page scripts

Acceptance Criteria

• ✅ Primary user flows are keyboard operable (skip link, tab navigation, Enter/Space on cards)
• ✅ Lighthouse accessibility score improves materially (focus indicators, semantic labels, ARIA)
• ✅ No obvious focus traps in modal/panel interactions (focus trap cycles correctly)

Lighthouse Impact

Audit	Before	After
Focus indicators	❌	✅ :focus-visible on all interactive elements
Skip link	❌	✅ Auto-injected skip-to-content
ARIA labels	⚠️ Partial	✅ Live regions, form validation, disclosure widgets
Color contrast	⚠️	✅ .text-muted bumped to AA compliance
Keyboard traps	—	✅ Modal focus trap with Tab cycling

Closes #35

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📥 Commits

Reviewing files that changed from the base of the PR and between 6eb1d2b and 44d7c3c.

📒 Files selected for processing (2)

• public/assets/css/accessibility.css
• public/assets/js/accessibility.js

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