feat(i18n): foundation — react-i18next, language switcher, server-persisted choice

Set up react-i18next with locale files auto-loaded per area (Vite glob), a compact
LanguageSwitcher, and language as a server-persisted preference (preferences.language)
mirrored to localStorage. On first login the default UI language is guessed from the
Google-reported locale (hu/en/de, else English). vite-env.d.ts types the build-time env.

frontend/src/main.tsx

@@ -5,6 +5,7 @@ import App from "./App";
 import ErrorBoundary from "./components/ErrorBoundary";
 import PrivacyPolicy from "./components/legal/PrivacyPolicy";
 import Terms from "./components/legal/Terms";
+import "./i18n";
 import "./index.css";
 
 const queryClient = new QueryClient({