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/providers/form.html.heex

@@ -15,10 +15,10 @@
       <p class="admin-callout-title">Get your API key from {@provider.name}:</p>
       <ol class="admin-callout-list">
         <li>
-          <a href={@provider.login_url} target="_blank" rel="noopener" class="admin-link">
+          <.external_link href={@provider.login_url} class="admin-link">
             Log in to {@provider.name}
-          </a>
-          (or <a href={@provider.signup_url} target="_blank" rel="noopener" class="admin-link">create a free account</a>)
+          </.external_link>
+          (or <.external_link href={@provider.signup_url} class="admin-link">create a free account</.external_link>)
         </li>
         <li :for={step <- @provider.setup_steps}>
           {raw(step)}