Support className prop on shadcn components

The shadcn components don't accept a className prop, so there's no way to override or extend their styling from the spec.

For example, the Card component wraps children in <CardContent className="flex flex-col gap-3">. That forced gap-3 breaks compact layouts like stat cards — but there's no escape hatch to override it without creating a full custom component.

Suggestion

Add an optional className: z.string().nullable() to each shadcn component's props schema and forward it to the root element via cn():

// catalog.ts
Card: {
  props: z.object({
    // ...existing props
    className: z.string().nullable(),
  }),
}

// components.tsx
Card: ({ props, children }) => (
  <Card className={cn(existingClasses, props.className)}>
    ...
  </Card>
)

This matches how shadcn/ui itself works — every component accepts className. It would let users fine-tune layout without needing to rewrite entire components.

Start by reviewing catalog.ts and components.tsx to identify each shadcn component's props schema and root element. Add the nullable className prop and forward it through cn() for every component; done means users can override or extend each component's existing styling without rewriting it.