feat: high-contrast theme

[AnnieIj]

Add AAA-compliant high-contrast theme variant via next-themes.

• Extract high-contrast CSS into app/styles/themes/high-contrast.css
• Pure black/white palette with 21:1 contrast ratio
• White borders, enhanced focus indicators, underlined links
• Visible scrollbars, print-friendly, respects reduced-motion
• 8 tests covering provider integration and theme isolation
• Imported in app/layout.tsx, registered in ThemeProvider

Closes #458

Pull Request

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)
• Style/UI changes
• Test updates
• CI/CD changes
• Security improvements

Related Issues

Closes #[issue-number]
Related to #[issue-number]

Changes Made

Files Modified

• [List specific files and changes made]
• [Include file paths and line numbers if relevant]

Key Changes

• [Describe the main changes and their impact]
• [Include any architectural decisions made]

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing completed
• Cross-browser testing (if applicable)
• Mobile responsiveness tested (if applicable)
• Accessibility testing completed
• Performance testing (if applicable)

Test Coverage

• New code is covered by tests
• Existing tests still pass
• Test coverage meets project standards

Screenshots/Videos

Minimum screenshots for UI/design PRs

• Desktop default state
• Mobile default state
• One stressed state: error, empty, loading, success, validation, open menu, or open modal
• Focus-visible screenshot for the primary interactive element (required when focus behavior changed)
• Open overlay screenshot for modal, drawer, popover, select, or date picker (required when applicable)
• Quick actions closed and open states (required for toolbar, FAB, or bottom-sheet action changes)
• Keyboard-open mobile screenshot and desktop sticky state (required for sticky action panel changes)

Before

After

Pre-submission Checklist

Code Quality

• Code follows project style guidelines
• Self-review completed
• Code is self-documenting
• No console errors or warnings
• No linting errors
• No TypeScript errors (if applicable)

Functionality

• All tests pass
• Feature works as expected
• No breaking changes introduced
• Performance impact assessed

Documentation

• README updated (if applicable)
• Code comments added where necessary
• API documentation updated (if applicable)
• Changelog updated (if applicable)

Security & Accessibility

• Security considerations addressed
• Accessibility standards met
• No sensitive data exposed
• Input validation implemented

Breaking Changes

Breaking Changes:

• [List any breaking changes]

Migration Steps:

• [Provide steps for users to migrate]

Additional Notes

Dependencies

• No new dependencies added
• Dependencies updated (list changes)
• Security vulnerabilities addressed

Performance Impact

• No performance impact
• Performance improved
• Performance impact documented

Browser/Device Support

• Tested on Chrome
• Tested on Firefox
• Tested on Safari
• Tested on Edge
• Tested on mobile devices

Labels

• feature - for new features
• bugfix - for bug fixes
• documentation - for documentation changes
• enhancement - for improvements
• breaking-change - for breaking changes
• frontend - for frontend changes
• backend - for backend changes
• ui/ux - for design changes

Reviewers

• Frontend team review
• Backend team review (if applicable)
• Design team review (if applicable)
• Security review (if applicable)

---

Note: Please ensure all checkboxes are completed before submitting this PR. This helps maintain code quality and speeds up the review process.

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[greatest0fallt1me]

Merged into main via admin resolver (-X theirs).

[greatest0fallt1me]

Nice work, everything checks out. Merged 🎉