Feature Parity 21: External Signer / HWI Flow

[johnny9]

Summary

Deliver external signer flow parity for transaction signing with hardware/external devices.

Complexity

• XL

Dependencies

• 18
• 16
• 10
• 02

Global Constraints

• All implementation lives under gui-qml/.
• No GUI runtime dependency on bitcoin/src/qt components. Reused logic must be moved/ported into gui-qml/.
• Do not change interfaces::Wallet or interfaces::Node in this version.
• Issue completion must satisfy DefinitionOfDone.md.

Scope

• Add signer detection surface using Node::listExternalSigners.
• Integrate send + PSBT flow for external signing path.
• Expose signer configuration guidance and unavailable-state handling.
• Support partial-sign and finalize loops.

Acceptance Criteria (DoD-Aligned)

1. User can detect available external signer and complete supported signing flow.
2. Missing signer, multiple signer, and signer error conditions are explicit and recoverable.
3. PSBT/external signer interoperability is validated end-to-end.

Required Tests

• C++ unit tests: Unit tests with signer mocks/stubs for detection and flow branching.
• QML QTest: QTests for signer state UI and action enablement.
• Python E2E (qml/test/testbridge.h): E2E flow with mocked external signer path including failure/retry behavior.

Implementation Notes

• Treat this as late-stage parity once PSBT core flow is stable.