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/package.json

@@ -13,9 +13,11 @@
     "@dnd-kit/utilities": "^3.2.2",
     "@tanstack/react-query": "^5.51.0",
     "clsx": "^2.1.1",
+    "i18next": "^23.11.5",
     "lucide-react": "^0.408.0",
     "react": "^18.3.1",
-    "react-dom": "^18.3.1"
+    "react-dom": "^18.3.1",
+    "react-i18next": "^14.1.2"
   },
   "devDependencies": {
     "@types/react": "^18.3.3",