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/vite-env.d.ts

@@ -0,0 +1,7 @@
+/// <reference types="vite/client" />
+
+interface ImportMetaEnv {
+  readonly VITE_APP_VERSION?: string;
+  readonly VITE_GIT_SHA?: string;
+  readonly VITE_BUILD_DATE?: string;
+}