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/account/components/AvatarUpload.tsx

@@ -32,7 +32,7 @@ export function AvatarUpload() {
     <div className="flex flex-col items-center gap-2">
       <label
         htmlFor={id}
-        className="cursor-pointer rounded-md border border-shoal px-3 py-1.5 text-sm text-snow hover:border-frost"
+        className="cursor-pointer rounded-ctl border border-shoal px-3 py-1.5 text-sm text-snow hover:border-frost"
       >
         {t('avatar.change')}
         <input