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/live/admin/settings.ex

@@ -592,14 +592,9 @@ defmodule BerrypodWeb.Admin.Settings do
       <p class="admin-section-desc admin-section-desc-flush">
         To accept payments, connect your Stripe account by entering your secret key.
         You can find it in your
-        <a
-          href="https://dashboard.stripe.com/apikeys"
-          target="_blank"
-          rel="noopener"
-          class="admin-link"
-        >
+        <.external_link href="https://dashboard.stripe.com/apikeys" class="admin-link">
           Stripe dashboard
-        </a>
+        </.external_link>
         under Developers &rarr; API keys.
       </p>