feat: add Dutch (nl) locale and fix missing translation keys across all locales

Add complete Dutch translation (2054 keys) with Netherlands flag in the
language selector. Also synchronize all existing locales so every language
has the same set of keys: added 29 missing keys to EN/RU/PT and 95 missing
keys to DE (moderation, analytics, CSS templates, backup, events).

frontend/src/components/common/LanguageSelector.tsx

@@ -38,11 +38,20 @@ const PTBRFlag: React.FC<{ className?: string }> = ({ className = "w-5 h-5" }) =
   </svg>
 );
 
+const NLFlag: React.FC<{ className?: string }> = ({ className = "w-5 h-5" }) => (
+  <svg className={className} viewBox="0 0 640 480" xmlns="http://www.w3.org/2000/svg">
+    <path fill="#AE1C28" d="M0 0h640v160H0z"/>
+    <path fill="#FFF" d="M0 160h640v160H0z"/>
+    <path fill="#21468B" d="M0 320h640v160H0z"/>
+  </svg>
+);
+
 const languages = [
   { code: 'en', name: 'English', Flag: GBFlag },
   { code: 'de', name: 'Deutsch', Flag: DEFlag },
   { code: 'ru', name: 'Русский', Flag: RUFlag },
   { code: 'pt', name: 'Português', Flag: PTBRFlag },
+  { code: 'nl', name: 'Nederlands', Flag: NLFlag },
 ];
 
 export const LanguageSelector: React.FC = () => {