test: add tests for NavbarDesktop

[Anujsinghdevx]

Description

Adds unit tests for NavbarDesktop to improve coverage and reliability. The tests validate navigation rendering, user menu interactions, task export functionality, auto-sync behavior, and external link handling. Test mocks were updated to reflect actual component behavior and avoid React DOM warnings

• Fixes: Add test suites for Frontend files to bump overall coverage #352

Checklist

• Ran npx prettier --write . (for formatting)
• Ran gofmt -w . (for Go backend)
• Ran npm test (for JS/TS testing)
• Added unit tests, if applicable
• Verified all tests pass
• Updated documentation, if needed

Additional Notes

No production code was modified; this PR focuses solely on improving test coverage.

[github-actions]

Thank you for opening this PR!

Before a maintainer takes a look, it would be really helpful if you could walk through your changes using GitHub's review tools.

Please take a moment to:

• Check the "Files changed" tab
• Leave comments on any lines for functions, comments, etc. that are important, non-obvious, or may need attention
• Clarify decisions you made or areas you might be unsure about and/or any future updates being considered.
• Finally, submit all the comments!

More information on how to conduct a self review:
https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/reviewing-proposed-changes-in-a-pull-request

This helps make the review process smoother and gives us a clearer understanding of your thought process.

Once you've added your self-review, we'll continue from our side. Thank you!