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>

lib/simpleshop_theme_web/user_auth.ex

@@ -257,12 +257,8 @@ defmodule SimpleshopThemeWeb.UserAuth do
   end
 
   @doc "Returns the path to redirect to after log in."
-  # the user was already logged in, redirect to admin dashboard
-  def signed_in_path(%Plug.Conn{assigns: %{current_scope: %Scope{user: %Accounts.User{}}}}) do
-    ~p"/admin"
-  end
-
-  def signed_in_path(_), do: ~p"/"
+  # Single-tenant: every user is the admin, always go to dashboard
+  def signed_in_path(_), do: ~p"/admin"
 
   @doc """
   Plug for routes that require the user to be authenticated.