fix: SSR error - check for browser before accessing navigator

- Add typeof window check in I18nContext useEffect
- Safely access navigator.language with optional chaining

src/i18n/I18nContext.tsx

@@ -22,13 +22,16 @@ export const I18nProvider = ({ children }: { children: ReactNode }) => {
     const [isInitialized, setIsInitialized] = useState(false);
 
     useEffect(() => {
+        // Only run on client side
+        if (typeof window === 'undefined') return;
+
         // Check for saved preference first
         const savedLocale = localStorage.getItem('locale') as Locale;
         if (savedLocale && (savedLocale === 'de' || savedLocale === 'en')) {
             setLocaleState(savedLocale);
         } else {
             // Auto-detect from browser: default to English, switch to German if detected
-            const browserLang = navigator.language.toLowerCase();
+            const browserLang = navigator.language?.toLowerCase() || 'en';
             if (browserLang.startsWith('de')) {
                 setLocaleState('de');
                 localStorage.setItem('locale', 'de');