add external link UX: icons, rel attributes, screen reader labels

New external_link component in core_components handles target="_blank",
rel="noopener noreferrer", external-link icon, and sr-only "(opens in
new tab)" text. Migrated admin providers form, settings (Stripe),
order tracking, onboarding setup links to use it. Fixed rel="noopener"
to "noopener noreferrer" on remaining links (email settings, product
show, core_components card radio). Added sr-only text to shop social
link cards and aria-label to page renderer tracking link.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod_web/components/shop_components/content.ex

@@ -431,6 +431,7 @@ defmodule BerrypodWeb.ShopComponents.Content do
               <.social_icon platform={link.platform} />
             </span>
             <span>{link.label}</span>
+            <span class="sr-only">(opens in new tab)</span>
           </a>
         <% end %>
       </div>