[P3][UX][planned] Fix accessibility: keyboard navigation, focus management, ARIA labels, landmarks

[xodapi]

Description
Audit revealed several accessibility gaps:

Findings

1. Keyboard navigation: most games (SchulteGrid, StroopTest, NBackTest, etc.) rely entirely on mouse/touch clicks. No keyboard support for core gameplay.
2. Focus management: AuthModal and FeedbackModal do not trap focus. Tab can reach elements behind modal. Focus not restored on close.
3. ARIA labels: most icon-only buttons (mobile menu toggle, close, theme toggle) lack aria-label. Only 3 aria-labels exist in the whole codebase.
4. Navigation landmarks: header nav uses with onClick navigators. Screen readers don't recognize these as navigation. Should be with /.
5. Skip-to-content: no skip link for keyboard users to bypass header.

Acceptance Criteria

1. Add keyboard event handlers to all game components (Tab + Enter/Space for primary actions)
2. Implement focus trap in AuthModal and FeedbackModal with restore on close
3. Add aria-label to all icon-only buttons
4. Replace nav buttons with / elements with role=navigation
5. Add skip-to-content link at top of App.tsx
6. Verify with axe DevTools or similar a11y scanner

Severity: Medium

• Modal focus escape is a WCAG failure
• No a11y testing in CI currently