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/page_renderer.ex

@@ -888,6 +888,7 @@ defmodule BerrypodWeb.PageRenderer do
                 target="_blank"
                 rel="noopener noreferrer"
                 class="order-detail-tracking-btn themed-button"
+                aria-label="Track parcel (opens in new tab)"
               >
                 Track parcel
               </a>