fs: consistently use fsbridge for {i,o}fstream

Part of #20744, but this can be done now, and will simplify the diff.

src/qt/psbtoperationsdialog.cpp

@@ -158,7 +158,7 @@ void PSBTOperationsDialog::saveTransaction() {
     if (filename.isEmpty()) {
         return;
     }
-    std::ofstream out(filename.toLocal8Bit().data(), std::ofstream::out | std::ofstream::binary);
+    fsbridge::ofstream out{filename.toLocal8Bit().data(), fsbridge::ofstream::out | fsbridge::ofstream::binary};
     out << ssTx.str();
     out.close();
     showStatus(tr("PSBT saved to disk."), StatusLevel::INFO);

src/qt/sendcoinsdialog.cpp

@@ -509,7 +509,7 @@ void SendCoinsDialog::sendButtonClicked([[maybe_unused]] bool checked)
             if (filename.isEmpty()) {
                 return;
             }
-            std::ofstream out(filename.toLocal8Bit().data(), std::ofstream::out | std::ofstream::binary);
+            fsbridge::ofstream out{filename.toLocal8Bit().data(), fsbridge::ofstream::out | fsbridge::ofstream::binary};
             out << ssTx.str();
             out.close();
             Q_EMIT message(tr("PSBT saved"), "PSBT saved to disk", CClientUIInterface::MSG_INFORMATION);

src/qt/walletframe.cpp

@@ -210,7 +210,7 @@ void WalletFrame::gotoLoadPSBT(bool from_clipboard)
             Q_EMIT message(tr("Error"), tr("PSBT file must be smaller than 100 MiB"), CClientUIInterface::MSG_ERROR);
             return;
         }
-        std::ifstream in(filename.toLocal8Bit().data(), std::ios::binary);
+        fsbridge::ifstream in{filename.toLocal8Bit().data(), std::ios::binary};
         data = std::string(std::istreambuf_iterator<char>{in}, {});
     }