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/index.css

@@ -19,6 +19,9 @@
   --text-hero: 3.8125rem;
   --text-hero--line-height: 1.05;
   --radius-panel: 10px;
+  /* Buttons, inputs and their containers. The landing CTAs hard-code the same value, so one
+     control cannot land at 6px while its neighbour in the same row lands at 10px. */
+  --radius-ctl: 10px;
 }
 
 /* The page accent follows the active ClickGui theme (useActiveTheme sets it on :root);