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/order_show.ex

@@ -134,15 +134,13 @@ defmodule BerrypodWeb.Admin.OrderShow do
         >
           <span class="admin-text-secondary"><.icon name="hero-truck-mini" class="size-4" /></span>
           <span class="admin-text-medium">{@order.tracking_number}</span>
-          <a
+          <.external_link
             :if={@order.tracking_url not in [nil, ""]}
             href={@order.tracking_url}
-            target="_blank"
-            rel="noopener"
             class="admin-link"
           >
-            Track shipment &rarr;
-          </a>
+            Track shipment
+          </.external_link>
         </div>
         <.order_timeline entries={@timeline} />
       </div>