Add DNS-based domain/DC auto-discovery

MainWindow.axaml.cs

@@ -29,6 +29,74 @@ public partial class MainWindow : Window
 
         RefreshConnectionHelp();
         RefreshActionHelp();
+
+        _ = AutoDiscoverAsync();
+    }
+
+    private async void Discover_Click(object? sender, RoutedEventArgs e)
+    {
+        DiscoverButton.IsEnabled = false;
+        SetStatus("Discovering domain and domain controller from DNS...");
+
+        try
+        {
+            var result = await DomainDiscovery.DiscoverAsync(TimeSpan.FromSeconds(10));
+
+            if (result.LdapHost is not null)
+            {
+                LdapHostBox.Text = result.LdapHost;
+            }
+
+            if (result.SearchBaseDn is not null)
+            {
+                SearchBaseBox.Text = result.SearchBaseDn;
+            }
+
+            if (result.Domain is not null)
+            {
+                var domainUpper = result.Domain.ToUpperInvariant();
+                var domainParts = result.Domain.Split('.');
+                var netbios = domainParts.Length > 0 ? domainParts[0].ToUpperInvariant() : domainUpper;
+                var userPlaceholder = $@"{netbios}\adminuser or adminuser@{result.Domain}";
+                BindUserBox.PlaceholderText = userPlaceholder;
+            }
+
+            SetStatus(result.Message ?? "Discovery did not return a result.");
+        }
+        catch (Exception ex)
+        {
+            SetStatus($"Discovery failed: {ex.Message}");
+        }
+        finally
+        {
+            DiscoverButton.IsEnabled = true;
+        }
+    }
+
+    private async Task AutoDiscoverAsync()
+    {
+        try
+        {
+            var result = await DomainDiscovery.DiscoverAsync(TimeSpan.FromSeconds(5));
+
+            if (result.LdapHost is not null && string.IsNullOrWhiteSpace(LdapHostBox.Text))
+            {
+                LdapHostBox.Text = result.LdapHost;
+            }
+
+            if (result.SearchBaseDn is not null && string.IsNullOrWhiteSpace(SearchBaseBox.Text))
+            {
+                SearchBaseBox.Text = result.SearchBaseDn;
+            }
+
+            if (result.LdapHost is not null || result.SearchBaseDn is not null)
+            {
+                SetStatus("Auto-discovered AD domain from DNS. Adjust if needed, then enter credentials.");
+            }
+        }
+        catch
+        {
+        }
     }
 
     private async void LoadCsv_Click(object? sender, RoutedEventArgs e)