fix(centralui): swallow ArgumentException in TransportImport upload step

OnFileSelectedAsync called TryLoadAsync with a null passphrase to peek
the manifest, but the outer `catch (Exception)` surfaced the expected
"Passphrase required for encrypted bundle" ArgumentException as a fatal
"Failed to read bundle" error -- blocking the user from ever advancing
to the passphrase step. Catch ArgumentException specifically and let
the wizard advance normally on the next click.

src/ScadaLink.CentralUI/Components/Pages/Design/TransportImport.razor.cs

@@ -126,6 +126,11 @@ public partial class TransportImport : ComponentBase
 
             await TryLoadAsync(passphrase: null);
         }
+        catch (ArgumentException)
+        {
+            // Encrypted bundle, no passphrase yet — expected. The wizard
+            // advances to the passphrase step when the user clicks Next.
+        }
         catch (Exception ex)
         {
             _errorMessage = $"Failed to read bundle: {ex.Message}";