Receive QR: enable save and share actions

[johnny9]

Gap

Payment request detail can copy the URI, but Share is disabled and there is no Save QR image action. Qt widgets support saving the QR image.

Work

Enable platform-appropriate share behavior and add a save-QR action with success/failure feedback. Keep copy URI and copy address behavior intact.

Code References

• qml/pages/wallet/PaymentRequestDetail.qml:143
• bitcoin/src/qt/receiverequestdialog.cpp:49