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/email_settings.ex

@@ -272,8 +272,9 @@ defmodule BerrypodWeb.Admin.EmailSettings do
                     :if={adapter.url}
                     href={adapter.url}
                     target="_blank"
-                    rel="noopener"
+                    rel="noopener noreferrer"
                     class="admin-link-subtle admin-adapter-link"
+                    aria-label={adapter.name <> " website (opens in new tab)"}
                   >
                     &nearr;
                   </a>