External signer signing: save or copy incomplete PSBT after partial signing

[johnny9]

Gap

When external signing succeeds but the transaction is still incomplete, QML treats the result as an error. Qt presents the partially signed PSBT so the user can copy or save it for additional signatures.

Work

After external signer approval, detect incomplete-but-signed PSBTs and offer copy/save PSBT fallback instead of a hard error. Coordinate with the PSBT operations and watch-only send work.

Code References

• qml/models/walletqmlmodel.cpp:1218
• qml/components/ExternalSignerReviewActions.qml:74
• bitcoin/src/qt/sendcoinsdialog.cpp:532
• bitcoin/src/qt/sendcoinsdialog.cpp:540