theme auth/setup pages and unify resets

Phase 7: add LoadTheme to base :browser pipeline so auth and setup
pages get theme settings. Update root.html.heex with .themed wrapper,
font preloads, layer declaration, and generated CSS injection.
Remove old data-theme JS toggle script.

Phase 8: upgrade admin/reset.css to a proper @layer reset matching
the shop reset structure. Remove dead theme toggle CSS rules.

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

lib/berrypod_web/router.ex

@@ -14,6 +14,7 @@ defmodule BerrypodWeb.Router do
     plug :put_secure_browser_headers
     plug :fetch_current_scope_for_user
     plug BerrypodWeb.Plugs.CountryDetect
+    plug BerrypodWeb.Plugs.LoadTheme
   end
 
   pipeline :api do