External signer send: gate approval on signer availability and show recovery

[johnny9]

Gap

QML shows the external signer approval action whenever the wallet has the external-signer flag, then discovers missing signer/path only after the user clicks. The signerNotFound signal argument is ignored by the review component.

Work

Surface signer availability in send review before approval, distinguish missing signer from signer failure, and route users to reconnect/check-device or signer path settings when appropriate.

Code References

• qml/pages/wallet/SendReview.qml:109
• qml/pages/wallet/MultipleSendReview.qml:256
• qml/components/ExternalSignerReviewActions.qml:74
• qml/models/walletqmlmodel.cpp:1170
• bitcoin/src/qt/sendcoinsdialog.cpp:204