Integrate QML app into the upstream build layout

[johnny9]

Parent: #40

Goal

Move the QML application from the external wrapper-repo build shape into the upstream Bitcoin Core build layout in a form suitable for merge review.

Scope

• Add a first-class upstream CMake option and target for the QML GUI.
• Place QML sources, resources, generated files, tests, and install rules in the expected upstream tree structure.
• Preserve shared GUI/core infrastructure needed by QML while avoiding unnecessary coupling to the existing Qt Widgets executable.
• Ensure the target works with local dependency builds and with the depends/Guix paths tracked separately.

Acceptance Criteria

• The QML app can be configured, built, tested, and installed from the upstream tree without the wrapper repo assumptions.
• CMake options clearly distinguish the QML GUI from the legacy Qt Widgets GUI.
• Build documentation reflects the new target names and expected developer workflow.
• Review-facing changes are separated from legacy Qt Widgets removal where practical.