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/admin/providers/index.html.heex

@@ -29,7 +29,7 @@
       <.button navigate={~p"/admin/providers/new?type=printify"}>
         Connect Printify
       </.button>
-      <.button navigate={~p"/admin/providers/new?type=printful"} class="admin-btn-outline">
+      <.button navigate={~p"/admin/providers/new?type=printful"} variant="outline">
         Connect Printful
       </.button>
     </div>