refactor: extract skip_link to shared ShopComponents module

Move the accessibility skip link from ThemeLive.PreviewPages to the
shared ShopComponents module for reuse on public storefront pages.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/simpleshop_theme_web/components/shop_components.ex

@@ -38,4 +38,18 @@ defmodule SimpleshopThemeWeb.ShopComponents do
     </div>
     """
   end
+
+  @doc """
+  Renders the skip link for keyboard navigation accessibility.
+
+  This is a standard accessibility pattern that allows keyboard users
+  to skip directly to the main content.
+  """
+  def skip_link(assigns) do
+    ~H"""
+    <a href="#main-content" class="skip-link">
+      Skip to main content
+    </a>
+    """
+  end
 end