extract setup wizard to dedicated /admin/setup page

Move the setup stepper out of the dashboard into its own LiveView.
Dashboard now redirects to setup when site isn't live, and shows
stats-only view once live. Also cleans up button component variant
handling, fixes alert CSS, and removes stale demo.html.

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

lib/berrypod_web/user_auth.ex

@@ -257,8 +257,9 @@ defmodule BerrypodWeb.UserAuth do
   end
 
   @doc "Returns the path to redirect to after log in."
-  # Single-tenant: every user is the admin, always go to dashboard
-  def signed_in_path(_), do: ~p"/admin"
+  def signed_in_path(_) do
+    if Berrypod.Settings.site_live?(), do: ~p"/admin", else: ~p"/admin/setup"
+  end
 
   @doc """
   Plug for routes that require the user to be authenticated.