Increase hero image max size to 3840x2160 in admin UI

Raised the maximum allowed hero image dimensions from 1920x1080 to 3840x2160 in the RegistryAdmin component, enabling support for higher-resolution uploads.

src/BirthList.Web/Components/Pages/RegistryAdmin.razor.cs

@@ -821,8 +821,8 @@ public partial class RegistryAdmin : ComponentBase
         {
             const long maxOriginalFileSize = 10 * 1024 * 1024; // 10 MB
             const long maxProcessedFileSize = 3 * 1024 * 1024; // 3 MB
-            const int maxHeroImageWidth = 1920;
+            const int maxHeroImageWidth = 3840;
-            const int maxHeroImageHeight = 1080;
+            const int maxHeroImageHeight = 2160;
 
             var file = e.File;