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>

test/simpleshop_theme_web/controllers/user_session_controller_test.exs

@@ -21,7 +21,7 @@ defmodule SimpleshopThemeWeb.UserSessionControllerTest do
       assert redirected_to(conn) == ~p"/"
 
       # Now do a logged in request and assert on the page content
-      conn = get(conn, ~p"/users/settings")
+      conn = get(conn, ~p"/admin/settings")
       response = html_response(conn, 200)
       assert response =~ user.email
     end
@@ -83,7 +83,7 @@ defmodule SimpleshopThemeWeb.UserSessionControllerTest do
       assert redirected_to(conn) == ~p"/"
 
       # Now do a logged in request and assert on the page content
-      conn = get(conn, ~p"/users/settings")
+      conn = get(conn, ~p"/admin/settings")
       response = html_response(conn, 200)
       assert response =~ user.email
     end
@@ -105,7 +105,7 @@ defmodule SimpleshopThemeWeb.UserSessionControllerTest do
       assert Accounts.get_user!(user.id).confirmed_at
 
       # Now do a logged in request and assert on the page content
-      conn = get(conn, ~p"/users/settings")
+      conn = get(conn, ~p"/admin/settings")
       response = html_response(conn, 200)
       assert response =~ user.email
     end