Update CI and workflows for the v31 QML merge branch

[johnny9]

Parent: #40

Goal

Update CI so the merge branch continuously validates the v31 QML build, test, and packaging paths.

Scope

• Update GitHub Actions to build the QML target on the v31 merge branch.
• Run QML unit tests, QML tests, and the functional GUI automation suite where practical.
• Add coverage for the depends/Guix path once that work is ready.
• Keep artifact generation aligned with the new target names and expected release packaging.
• Revisit the legacy bitcoind version used by wallet migration tests and document the intended policy.

Acceptance Criteria

• CI builds the QML app against v31 on supported host runners.
• Relevant test suites run in CI with clear failure output.
• Artifact workflow names and uploaded binaries match the merge-branch target names.
• CI dependencies are explicit and do not hide missing depends/Guix packaging work.