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/i18n/index.ts

@@ -0,0 +1,58 @@
+import i18n from "i18next";
+import { initReactI18next } from "react-i18next";
+
+// Endonyms (shown as-is regardless of the active language — the convention for language pickers).
+export const LANGUAGES = [
+  { code: "en", label: "English" },
+  { code: "hu", label: "Magyar" },
+  { code: "de", label: "Deutsch" },
+] as const;
+export type LangCode = (typeof LANGUAGES)[number]["code"];
+const SUPPORTED = LANGUAGES.map((l) => l.code) as readonly string[];
+export const LANG_KEY = "siftlode.lang";
+
+// Auto-load every locale file (locales/<lang>/<area>.json) and merge each <area> under one
+// `translation` namespace, so components use t("area.key") and adding an area needs no wiring.
+const mods = import.meta.glob("./locales/*/*.json", { eager: true }) as Record<
+  string,
+  { default: Record<string, unknown> }
+>;
+
+const resources: Record<string, { translation: Record<string, unknown> }> = {};
+for (const [path, mod] of Object.entries(mods)) {
+  const m = path.match(/\/locales\/([a-z]{2})\/([a-zA-Z0-9_]+)\.json$/);
+  if (!m) continue;
+  const [, lang, area] = m;
+  (resources[lang] ??= { translation: {} }).translation[area] = mod.default;
+}
+
+export function isSupported(code: string | null | undefined): code is LangCode {
+  return !!code && SUPPORTED.includes(code);
+}
+
+// Initial language before the server preference is known: stored choice, else the browser
+// language, else English. (After login, App adopts the server-persisted preference.)
+export function detectInitialLang(): LangCode {
+  const stored = localStorage.getItem(LANG_KEY);
+  if (isSupported(stored)) return stored;
+  const nav = (navigator.language || "en").slice(0, 2).toLowerCase();
+  return isSupported(nav) ? nav : "en";
+}
+
+const initial = detectInitialLang();
+void i18n.use(initReactI18next).init({
+  resources,
+  lng: initial,
+  fallbackLng: "en",
+  interpolation: { escapeValue: false },
+  returnNull: false,
+});
+document.documentElement.lang = initial;
+
+export function setLanguage(code: LangCode): void {
+  void i18n.changeLanguage(code);
+  localStorage.setItem(LANG_KEY, code);
+  document.documentElement.lang = code;
+}
+
+export default i18n;