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/product_show.ex

@@ -102,11 +102,12 @@ defmodule BerrypodWeb.Admin.ProductShow do
           :if={provider_edit_url(@product)}
           href={provider_edit_url(@product)}
           target="_blank"
-          rel="noopener"
+          rel="noopener noreferrer"
           class="admin-btn admin-btn-ghost admin-btn-sm"
         >
           Edit on {provider_label(@product)}
           <.icon name="hero-arrow-top-right-on-square-mini" class="size-4" />
+          <span class="sr-only">(opens in new tab)</span>
         </.link>
         <.link
           navigate={~p"/products/#{@product.slug}"}