feat(frontend): unify control radius on one token

Buttons and inputs in the kit used rounded-md (6px) while the landing CTAs
hard-coded 0.6rem (9.6px), so a form row could mix three different corners.
Add --radius-ctl (10px, matching --radius-panel), point every interactive
control at it via rounded-ctl and make the CTAs consume the same variable.

Drop the unused ButtonAnchor helper.

frontend/src/features/showcase/components/CopyToSlot.tsx

@@ -56,7 +56,7 @@ export function CopyToSlot({ listingId }: { listingId: string }) {
         </Notice>
       ) : null}
       {error ? <Notice tone="error">{error}</Notice> : null}
-      <div role="group" aria-label={t('copy.heading')} className="inline-flex overflow-hidden rounded-md border border-shoal">
+      <div role="group" aria-label={t('copy.heading')} className="inline-flex overflow-hidden rounded-ctl border border-shoal">
         {SLOTS.map((slot) => (
           <button
             key={slot}