Improve className typing

Currently all components have `className` typed as `string | undefined` since it is an optional prop with no default. To deal with this we are coercing `className` to be strict string via: ```[`${className}`]: className```

the `classnames` library can print the value of `className` like so: `[className]: className` however it requires that `className` be strictly `string` type to use that syntax.

If we change `className` to not be optional and have a default value of `''` the type would be just `string` and we could use the simpler syntax in `classnames` function calls

Review the component prop definitions and the existing classnames call sites that coerce className with template literals. Determine how each component supplies a default empty string and confirm the className type remains consistent across components. Update the affected usage and verify that TypeScript checks pass without the coercion.

Written by the indexing model from the issue text.