fix(centralui): show Next button after encrypted-bundle upload

Step 1's Next button was wrapped in `@if (_session is not null)`, which
hid it for encrypted bundles where the first LoadAsync call legitimately
leaves _session null until the passphrase is supplied at Step 2.
Trigger the Next-button region on `_bundleBytes is not null` instead,
showing a placeholder notice when the manifest isn't decrypted yet so
the user has a visible affordance to advance to the passphrase step.

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

@@ -103,6 +103,8 @@
                 <div class="text-muted small fst-italic">Reading bundle…</div>
             }
 
+            @if (_bundleBytes is not null && _errorMessage is null)
+            {
                 @if (_session is not null)
                 {
                     <dl class="row small mt-3" data-testid="manifest-summary">
@@ -133,6 +135,14 @@
                             }
                         </dd>
                     </dl>
+                }
+                else
+                {
+                    <div class="alert alert-info mt-3" data-testid="encrypted-bundle-notice">
+                        <strong>Encrypted bundle uploaded.</strong>
+                        Click <strong>Next</strong> to enter the passphrase.
+                    </div>
+                }
 
                 <div class="d-flex justify-content-end mt-3">
                     <button class="btn btn-primary" @onclick="GoFromUploadAsync">Next</button>