classNames Utility

Implement solve(...args) — a clsx-style utility that builds a class
name string from its arguments:

• strings — included as-is (skip falsy).
• objects — keys whose values are truthy are included.
• arrays — recursively flattened.

Returns a single space-separated string of active class names.

solve('foo', 'bar')              // 'foo bar'
solve('foo', { bar: true })      // 'foo bar'
solve({ foo: true, bar: false }) // 'foo'
solve(['a', ['b', 'c']])         // 'a b c'