fix(app): collapse closed preparation scenario editor

[gcalpay]

Summary

Collapse the ML Preparation scenario editor Loader after its nested editor is
closed, preventing the former editor height from leaving a large blank region
before the Outputs and Quality diagnostics sections.

Changes

• bind the scenario Loader preferred height to the loaded editor height
• reset the preferred height to zero when the editor is unloaded
• add focused layout regression coverage at representative desktop sizes

Scope

No scientific, controller, schema, persistence, validation, planning,
execution or stale-result behavior changed.

Verification

• focused Preparation QML tests passed
• QML source checks passed
• manual opening and closing of the scenario editor confirmed the gap is removed