Address code review comments

- Fix NavBar.tsx to use desktop padding value instead of mobile
- Add explicit string return type to defaultFocusOutline export
- Add explicit string return type to applyButtonVariantStyles export
- Evaluate snapshot tests: keeping them as they provide value for
  structural validation and will stabilize after styled-components
  migration is complete

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: OpenStaxClaude