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.

backend/app/auth.py

@@ -151,6 +151,13 @@ async def callback(
     user.display_name = userinfo.get("name")
     user.avatar_url = userinfo.get("picture")
     user.role = "admin" if email in settings.admin_email_set else "user"
+    # Default UI language from the Google-reported locale on first login (if the user hasn't
+    # picked one yet); unsupported locales fall back to English.
+    prefs = dict(user.preferences or {})
+    if not prefs.get("language"):
+        loc = (userinfo.get("locale") or "").split("-")[0].lower()
+        prefs["language"] = loc if loc in {"en", "hu", "de"} else "en"
+        user.preferences = prefs
     db.flush()
 
     tok = user.token or OAuthToken(user=user)