feat(app): label watch-only send flow and export unsigned PSBT

Send already refused to enable Confirm for watch-only accounts (Signed
false), but the App had no way to get the resulting PSBT out to sign
elsewhere — only the CLI printed it. Adds a base64 PSBT box + copy button
in the Send summary card, and a visible warning banner explaining the
wallet cannot sign, on both desktop and mobile layouts.

src/App/ViewModels/MainWindowViewModel.cs

@@ -201,6 +201,8 @@ public partial class MainWindowViewModel : ViewModelBase
         _lastTransactions = null;
         _pendingSend = null;
         HasPendingSend = false;
+        PendingPsbtBase64 = "";
+        OnPropertyChanged(nameof(IsWatchOnlyAccount));
         History.Clear();
         Contacts.Clear();
         SelectedContactInList = null;