add admin products list and detail pages

Read-mostly admin views for synced products: filterable/sortable list
with inline visibility toggle, and detail page with images grid,
variants table, storefront controls form, and provider edit links.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/simpleshop_theme_web/components/layouts/admin.html.heex

@@ -61,6 +61,14 @@
               <.icon name="hero-shopping-bag" class="size-5" /> Orders
             </.link>
           </li>
+          <li>
+            <.link
+              navigate={~p"/admin/products"}
+              class={admin_nav_active?(@current_path, "/admin/products")}
+            >
+              <.icon name="hero-cube" class="size-5" /> Products
+            </.link>
+          </li>
           <li>
             <.link
               href={~p"/admin/theme"}