fix signed_in_path to always redirect to /admin after login

Single-tenant app: every user is the admin. The previous pattern
match on conn.assigns.current_scope didn't work for first-time
logins because the scope isn't assigned to the conn yet at that
point, causing the fallback to ~p"/" instead of ~p"/admin".

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

test/simpleshop_theme_web/live/auth/confirmation_test.exs

@@ -64,7 +64,7 @@ defmodule SimpleshopThemeWeb.Auth.ConfirmationTest do
       assert Accounts.get_user!(user.id).confirmed_at
       # we are logged in now
       assert get_session(conn, :user_token)
-      assert redirected_to(conn) == ~p"/"
+      assert redirected_to(conn) == ~p"/admin"
 
       # log out, new conn
       conn = build_conn()

test/simpleshop_theme_web/live/auth/login_test.exs

@@ -56,7 +56,7 @@ defmodule SimpleshopThemeWeb.Auth.LoginTest do
 
       conn = submit_form(form, conn)
 
-      assert redirected_to(conn) == ~p"/"
+      assert redirected_to(conn) == ~p"/admin"
     end
 
     test "redirects to login page with a flash error if credentials are invalid", %{

test/simpleshop_theme_web/live/auth/registration_test.exs

@@ -25,7 +25,7 @@ defmodule SimpleshopThemeWeb.Auth.RegistrationTest do
         conn
         |> log_in_user(user_fixture())
         |> live(~p"/users/register")
-        |> follow_redirect(conn, ~p"/")
+        |> follow_redirect(conn, ~p"/admin")
 
       assert {:ok, _conn} = result
     end