feat(ui): enhance accent color readability with WCAG contrast compliance

Add WCAG contrast ratio calculation functions to ensure accent colors
are readable against background colors. When contrast ratio falls below
3.0, the accent color is automatically shifted toward better visibility.

- Add _wcagLuminance, _contrastRatio, _shiftLightness functions
- Add ensureReadableAccent function with 10-step adjustment limit
- Introduce --ffa-accent-readable CSS variable for compliant colors
- Update UI components (cards, buttons, icons, tabs) to use readable accent
- Bump version to 3.5.5

This improves accessibility for users with visual impairments and
ensures better readability across various theme combinations.

Author: ffainy