Default localization to "de-DE" and refactor resources

Added a new `Model` class in the `EnvelopeGenerator.Domain.Resources` namespace as a marker for `IStringLocalizer` resource lookup. Updated `EnvelopeSenderPage.razor` to use the new `Model` class for localization.

Modified `LocalizationController` to default to "de-DE" when no culture is specified or found. Refactored localization setup in `Program.cs` to use a fixed default culture of "de-DE" and persist culture via cookies. Removed dynamic culture setup and `QueryStringRequestCultureProvider`.

Performed minor cleanup in `LocalizationController` and `Program.cs` to align with the new localization approach.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Program.cs

@@ -410,16 +410,16 @@ try
     // Set CORS policy
     app.UseCors("AllowSpecificOriginsPolicy");
 
-    // Localization
-    string[] supportedCultureNames = ["de-DE", "en-US"];
-    IList<CultureInfo> list = [.. supportedCultureNames.Select(cn => new CultureInfo(cn))];
-    var cultureInfo = list.FirstOrDefault() ?? throw new InvalidOperationException("There is no supported culture.");
+    // Localization (fixed to German for now; persisted via cookie)
+    var defaultCulture = new CultureInfo("de-DE");
+    IList<CultureInfo> list = [defaultCulture];
     var requestLocalizationOptions = new RequestLocalizationOptions
     {
+        DefaultRequestCulture = new RequestCulture(defaultCulture),
         SupportedCultures = list,
-        SupportedUICultures = list
+        SupportedUICultures = list,
+        RequestCultureProviders = [new CookieRequestCultureProvider()]
     };
-    requestLocalizationOptions.RequestCultureProviders.Add(new QueryStringRequestCultureProvider());
     app.UseRequestLocalization(requestLocalizationOptions);
 
     app.UseHttpsRedirection();