Constant BUTTON_BASE 

pub const BUTTON_BASE: &str = "inline-flex items-center justify-center gap-2 whitespace-nowrap rounded-[var(--control-radius)] text-sm \
                               font-medium transition-all cursor-pointer disabled:pointer-events-none disabled:opacity-50 \
                               [&_svg]:pointer-events-none [&_svg:not([class*='size-'])]:size-4 shrink-0 [&_svg]:shrink-0 \
                               outline-none focus-visible:border-ring focus-visible:ring-ring/50 focus-visible:ring-[3px] \
                               aria-invalid:ring-accent-error/20 aria-invalid:border-accent-error";

Base classes shared by every button variant. Split out from the variant enum so the TS codegen can emit it verbatim and button_classes fuses it last-wins.