Update gui-qml to Bitcoin Core v31

[johnny9]

Parent: #40

Goal

Update the Bitcoin Core submodule/base to v31 and make the QML application build and run against the v31 interfaces.

Scope

• Update the Bitcoin Core submodule or merge base to the selected v31 reference.
• Resolve C++ API, CMake target, resource, and translation build fallout from the v31 update.
• Re-run the QML unit, QML, and functional test suites and track any regressions.
• Document any v31-specific compatibility changes that affect the merge branch.

Acceptance Criteria

• The merge branch points at the selected v31 base.
• The QML app configures and builds against v31.
• Existing QML unit/QML tests pass, or remaining failures are filed as focused follow-up issues.
• Functional test blockers caused by the v31 update are identified and either fixed or explicitly tracked.