[Testing] Enhanced Test Coverage for Calendar Features

[seppelz]

Description\nAdd comprehensive test coverage for recently added calendar features.\n\n## Test Areas\n### Keyboard Navigation\n- [ ] Arrow key navigation\n- [ ] Enter/Space selection\n- [ ] Focus management\n- [ ] Visual feedback\n\n### Holiday Display\n- [ ] Color mapping for different holiday types\n- [ ] Gradient display for shared holidays\n- [ ] Text visibility and contrast\n- [ ] Holiday type mapping\n\n### Multi-User Features\n- [ ] Person-specific colors\n- [ ] Shared vacation display\n- [ ] Visual distinction between users\n\n### Accessibility\n- [ ] Keyboard interaction\n- [ ] ARIA attributes\n- [ ] Color contrast\n- [ ] Focus management\n\n## Technical Notes\n- Use Jest and React Testing Library\n- Add integration tests for complex interactions\n- Include visual regression tests for holiday colors\n- Add accessibility tests with jest-axe\n\n## Related Issues\n- #51 (Keyboard Navigation Enhancements)\n- #33 (Test Coverage Strategy)\n\n## Priority\nHigh