consolidate settings into single admin page

Merge shop status, payments, products (Printify), account (email/password),
and advanced (dashboard/error tracker links) into /admin/settings. Simplify
Auth.Settings to a redirector for /users/settings and confirm-email tokens.
Remove Providers from sidebar nav. Update all redirects and tests.

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

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

@@ -61,14 +61,6 @@
               <.icon name="hero-paint-brush" class="size-5" /> Theme
             </.link>
           </li>
-          <li>
-            <.link
-              navigate={~p"/admin/providers"}
-              class={admin_nav_active?(@current_path, "/admin/providers")}
-            >
-              <.icon name="hero-cube" class="size-5" /> Providers
-            </.link>
-          </li>
           <li>
             <.link
               navigate={~p"/admin/settings"}