fix: remove double-pop from BuildingTransactionDialog cancel

The dialog's cancel button popped the navigator itself and then invoked
onCancel, while callers' onCancel handlers also popped, causing a double
pop. Drop the internal pop so the caller owns the dismissal, and add the
missing pop to Step4View's onCancel.

Author: sneurlax

lib/pages/exchange_view/exchange_step_views/step_4_view.dart

@@ -270,6 +270,7 @@ class _Step4ViewState extends ConsumerState<Step4View> {
               isSpark: wallet is FiroWallet && !firoPublicSend,
               onCancel: () {
                 wasCancelled = true;
+                Navigator.of(context).pop();
               },
             );
           },

lib/pages/send_view/sub_widgets/building_transaction_dialog.dart

@@ -145,7 +145,6 @@ class _RestoringDialogState extends ConsumerState<BuildingTransactionDialog> {
                               style: STextStyles.itemSubtitle12(context),
                             ),
                             onPressed: () {
-                              Navigator.of(context).pop();
                               onCancel.call();
                             },
                           ),
@@ -172,7 +171,6 @@ class _RestoringDialogState extends ConsumerState<BuildingTransactionDialog> {
                     style: STextStyles.itemSubtitle12(context),
                   ),
                   onPressed: () {
-                    Navigator.of(context).pop();
                     onCancel.call();
                   },
                 ),