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/live/auth/registration.ex

@@ -32,7 +32,7 @@ defmodule BerrypodWeb.Auth.Registration do
             phx-mounted={JS.focus()}
           />
 
-          <.button phx-disable-with="Creating account..." class="admin-btn-primary w-full">
+          <.button phx-disable-with="Creating account..." variant="primary" class="w-full">
             Create an account
           </.button>
         </.form>