Validate culture cookie against supported languages

Changed CultureMiddleware to check if the culture cookie value is in the list of supported languages, not just for null or empty. This ensures that only valid, supported culture values are accepted from the cookie; otherwise, the middleware falls back to Accept-Language or default logic.

EnvelopeGenerator.Web/Middleware/CultureMiddleware.cs

@@ -22,7 +22,7 @@ public class CultureMiddleware
         var cookieName = CookieRequestCultureProvider.DefaultCookieName;
         var cookieValue = context.Request.Cookies[cookieName];
 
-        if (string.IsNullOrEmpty(cookieValue))
+        if (!_cultures.Languages.Contains(cookieValue))
         {
             var requestCulture = context.GetCultureByAcceptLanguage()?.Name;
             var culture = _cultures.GetOrDefault(requestCulture);