fix(ui): fix class reactivity and add new cn utility for classes

[dev-rb]

Summary

This PR adds clsx as a dependency (although we probably could just inline it since it's small) and introduces a new cn utility for writing complicated classes and then piping them through twMerge to be merged.

Also:

• Fixes an issue where the Button component's class property was not reactive
• Removes classList use from Button in favor of using the cn utility. This also removes the need for using ! (important) after certain classes

Screenshots, GIFs, and Videos

[github-actions]

Preview: https://rahul-fix-core-ui-class-handli-8g871i.preview.macro.com/app (c7138ea)

[claude]

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Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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[claude]

Code review

I found one issue that should be addressed:

Missing tests

The CLAUDE.md requires tests for changes, especially bug fixes:

Testing - Write tests for your changes. When fixing bugs or regressions, identify the issue with a test before fixing it.

This PR fixes a class reactivity bug in the Button component but does not include any tests to:

• Verify the bug fix works correctly
• Prevent regression of the reactivity issue
• Test the new cn utility function

Please add tests for:

1. The Button component's class reactivity (to verify the bug is fixed)
2. The new cn utility function (to ensure proper class merging behavior)

[sedson]

i have seen the truth