Fix FOUC and improve Web UI i18n experience

- Fix Flash of Unstyled Content (FOUC) by hiding page until translations load
  - Hide body initially with visibility:hidden
  - Show content after translations are applied to avoid English flash
- Fix authenticated indicator layout with white-space:nowrap
- Add "●" symbol to translation files to prevent disappearance on reload
- Enable network access by binding server to 0.0.0.0 in test config
- Simplify test config API keys to single entry

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

server/src/wled_controller/static/locales/ru.json

@@ -5,7 +5,7 @@
   "locale.change": "Изменить язык",
   "auth.login": "Войти",
   "auth.logout": "Выйти",
-  "auth.authenticated": "Авторизован",
+  "auth.authenticated": "● Авторизован",
   "auth.title": "Вход в WLED Контроллер",
   "auth.message": "Пожалуйста, введите ваш API ключ для аутентификации и доступа к WLED Контроллеру Экрана.",
   "auth.label": "API Ключ:",