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/controllers/user_session_controller_test.exs

@@ -18,7 +18,7 @@ defmodule SimpleshopThemeWeb.UserSessionControllerTest do
         })
 
       assert get_session(conn, :user_token)
-      assert redirected_to(conn) == ~p"/"
+      assert redirected_to(conn) == ~p"/admin"
 
       # Now do a logged in request and assert on the page content
       conn = get(conn, ~p"/admin/settings")
@@ -39,7 +39,7 @@ defmodule SimpleshopThemeWeb.UserSessionControllerTest do
         })
 
       assert conn.resp_cookies["_simpleshop_theme_web_user_remember_me"]
-      assert redirected_to(conn) == ~p"/"
+      assert redirected_to(conn) == ~p"/admin"
     end
 
     test "logs the user in with return to", %{conn: conn, user: user} do
@@ -80,7 +80,7 @@ defmodule SimpleshopThemeWeb.UserSessionControllerTest do
         })
 
       assert get_session(conn, :user_token)
-      assert redirected_to(conn) == ~p"/"
+      assert redirected_to(conn) == ~p"/admin"
 
       # Now do a logged in request and assert on the page content
       conn = get(conn, ~p"/admin/settings")
@@ -99,7 +99,7 @@ defmodule SimpleshopThemeWeb.UserSessionControllerTest do
         })
 
       assert get_session(conn, :user_token)
-      assert redirected_to(conn) == ~p"/"
+      assert redirected_to(conn) == ~p"/admin"
       assert Phoenix.Flash.get(conn.assigns.flash, :info) =~ "User confirmed successfully."
 
       assert Accounts.get_user!(user.id).confirmed_at