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/components/LanguageSwitcher.tsx

@@ -95,7 +95,7 @@ export default function LanguageSwitcher({
           className="relative p-2 rounded-lg text-muted hover:text-fg hover:bg-card transition"
         >
           <Globe className="w-5 h-5" />
-          {/* Current-language badge (HU/EN/DE) so the active language reads at a glance without
+          {/* Current-language badge (HU/EN) so the active language reads at a glance without
               opening the menu. The ring matches the page background to detach it from the icon. */}
           <span className="absolute -bottom-1 -right-1 text-[8px] font-bold leading-none px-1 py-[1px] rounded bg-accent text-accent-fg ring-2 ring-bg">
             {current.code.toUpperCase()}