Feature Parity 18: PSBT Import/Export Flow

[johnny9]

Summary

Implement PSBT export from send flow and PSBT import/sign/finalize pathways.

Complexity

• XL

Dependencies

• 16
• 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 create/export PSBT actions (clipboard + file).
• Add PSBT import path (file + clipboard) and decode/validation UX.
• Support sign/finalize/broadcast paths using wallet interface methods.
• Integrate with external signer readiness states where relevant.

Acceptance Criteria (DoD-Aligned)

1. User can export unsigned PSBT from transaction review.
2. User can import PSBT, inspect status, sign if possible, and finalize/broadcast when complete.
3. Large/invalid PSBT inputs fail gracefully with actionable messages.

Required Tests

• C++ unit tests: Unit tests for PSBT parsing/sign/finalize adapters and error handling.
• QML QTest: QTests for import/export dialog states and action enablement.
• Python E2E (qml/test/testbridge.h): E2E flow covering PSBT export, re-import, sign/finalize, and broadcast.

Implementation Notes

• Do not reuse qt/psbtoperationsdialog.*; implement gui-qml-native model + UI.