feat(a11y): Add high contrast mode support (#4853)

[veerababu1729]

Fixes #4853

Description

This PR implements the High Contrast Mode feature requested in issue #4853, which is a follow-up to the dark mode project.

How This Fixes Issue #4853

The issue requested:

1. ✅ Design: High contrast versions of Openverse light and dark mode designs
2. ✅ Design: User interface control for the high contrast option
3. ✅ Implementation: Color variables and root-element class context for high contrast
4. ✅ Implementation: Controls for high-contrast variations

This implementation provides:

• High contrast CSS variables for both light and dark modes with enhanced contrast ratios (WCAG 1.4.6 compliant - 7:1 ratio)
• Automatic detection of OS-level prefers-contrast: more media query
• A new "Contrast" dropdown in the footer with options: Normal, High, System
• Feature flag high_contrast_ui_toggle for controlled rollout

Files Changed

File	Changes
src/styles/tailwind.css	+250 lines of high contrast CSS variables
src/composables/use-high-contrast.ts	New composable (similar to use-dark-mode.ts)
src/components/VContrastSelect/VContrastSelect.vue	New UI dropdown component
src/stores/ui.ts	Added ContrastMode type, state, and actions
feat/feature-flags.json	Added high_contrast_ui_toggle flag
shared/types/cookies.ts	Added contrastMode to cookie state
i18n/data/en.json5	Added translation keys
src/components/VFooter/VFooter.vue	Integrated VContrastSelect
src/app.vue	Applied contrast CSS class to body
src/error.vue	Applied contrast CSS class to body

Testing Instructions

1. Enable the feature flag high_contrast_ui_toggle
2. Navigate to any page and scroll to the footer
3. Use the new "Contrast" dropdown to switch between Normal/High/System
4. Verify enhanced contrast is applied (stronger borders, pure black/white text)
5. Test with OS high contrast mode enabled (Settings → Accessibility → High Contrast)

Checklist

• My pull request has a descriptive title
• My pull request targets the main branch
• My commit messages follow the conventional commits format
• I have tested my changes locally
• I have added/updated tests as needed