[web] Render "Try for free instead" CTA as link, not button (#34098)

* [web] Render "Try for free instead" CTA as link, not button

Design QA wants the "Try for free instead" CTAs on the pricing and
interstitial pages styled as marketing links (`link-monospace link-lg`)
rather than the current `btn-ghost` button. Add a `link` button type to
the `plans-cta` mixin that drops the `btn` class and applies the link
classes, and set `buttonType: 'link'` on the six `try_for_free_instead`
CTAs (plans-individual, plans-student, interstitial-payment).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Make link smaller

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
GitOrigin-RevId: f911698a9bfa19f8180e58edb3cebcea90468cbd

services/web/frontend/stylesheets/pages/website-redesign.scss

@@ -105,6 +105,10 @@
       }
     }
 
+    &.link-sm {
+      @include body-sm;
+    }
+
     &.btn:hover {
       text-decoration: none;
     }