Remove the legacy Qt Widgets GUI from the merge branch build path

[johnny9]

Parent: #40

Goal

Make the merge branch explicit about removing or disabling the legacy Qt Widgets GUI from the relevant build/release path while preserving shared infrastructure still needed by QML.

Scope

• Decide whether legacy Qt Widgets sources are removed from the branch, disabled by default, or excluded only from release targets.
• Split shared Qt/core infrastructure from Widgets-only code where QML still depends on it.
• Update CMake options, install rules, packaging, and docs to make the QML GUI the intended GUI target for the merge branch.
• Keep this reviewable independently from the initial QML target integration when possible.

Acceptance Criteria

• The merge branch no longer builds or packages the legacy Qt Widgets GUI in the intended release path.
• QML still has access to any shared models/resources/helpers it legitimately needs.
• Build options and documentation make the relationship between QML and legacy Widgets code unambiguous.
• Any remaining Widgets-only source retained temporarily has a documented reason and follow-up plan.