Update culture defaults and improve second-hand dropdown

- Change default request culture to "nl-BE" and reorder supported cultures; remove "nl-NL"
- Refactor RegistryAdmin second-hand preference to use a string property for better nullable handling in the dropdown

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

@@ -120,7 +120,7 @@ else
                         </div>
                         <div class="col-md-3">
                             <label class="form-label">@L["RegistryAdmin.SecondHandPreference"]</label>
-                            <InputSelect class="form-select" @bind-Value="ItemModel.PreferSecondHand">
+                            <InputSelect class="form-select" @bind-Value="PreferSecondHandString">
                                 <option value="">@L["RegistryAdmin.SecondHandOptional"]</option>
                                 <option value="true">@L["RegistryAdmin.PreferSecondHand"]</option>
                                 <option value="false">@L["RegistryAdmin.NewOnly"]</option>

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

@@ -26,6 +26,21 @@ public partial class RegistryAdmin : ComponentBase
 
     protected RegistrySettingsEditModel SettingsModel { get; } = new();
     protected RegistryItemEditModel ItemModel { get; private set; } = new();
+    protected string PreferSecondHandString
+    {
+        get => ItemModel.PreferSecondHand switch
+        {
+            null => "",
+            true => "true",
+            false => "false"
+        };
+        set => ItemModel.PreferSecondHand = value switch
+        {
+            "true" => true,
+            "false" => false,
+            _ => null
+        };
+    }
     protected IReadOnlyList<RegistryItemEditModel> Items { get; private set; } = [];
     protected IReadOnlyList<RegistryItemCategoryEditModel> ItemCategories { get; private set; } = [];
     protected string? NewCategoryName { get; set; }

src/BirthList.Web/Features/Localization/LocalizationService.cs

@@ -8,9 +8,8 @@ internal sealed class LocalizationService(UserManager<ApplicationUser> userManag
 {
     private static readonly HashSet<string> SupportedCultureNames =
     [
-        "en",
-        "nl-NL",
         "nl-BE",
+        "en",
         "fr-FR",
         "qps-Ploc"
     ];

src/BirthList.Web/Program.cs

@@ -149,8 +149,8 @@ builder.Services.AddLocalization(options =>
 
 var supportedCultures = new List<CultureInfo>
 {
-    new("en"),
     new("nl-BE"),
+    new("en"),
     new("fr-FR")
 };
 
@@ -161,7 +161,7 @@ if (builder.Environment.IsDevelopment())
 
 builder.Services.Configure<RequestLocalizationOptions>(options =>
 {
-    options.DefaultRequestCulture = new RequestCulture("en");
+    options.DefaultRequestCulture = new RequestCulture("nl-BE");
     options.SupportedCultures = supportedCultures;
     options.SupportedUICultures = supportedCultures;
     options.RequestCultureProviders =