feat(i18n): retire the German interface language

Only English and Hungarian remain. Drops the de locale files, the LANGUAGES
entry, the Google-locale mapping in auth, and the German welcome-message
template. A legacy "de" preference falls back to English gracefully. Content-
language detection and subtitle languages are untouched (media, not UI).

frontend/src/i18n/index.ts

@@ -6,7 +6,6 @@ import { LS } from "../lib/storage";
 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[];