Refactor EditStaff::apply() to use m_orgStaff for undo stack access

[CubikingChill]

Use m_orgStaff->score()->undoStack() instead of m_staff->score()->undoStack() for better code clarity and semantic correctness.

While both references point to the same undo stack (since m_staff and m_orgStaff share the same Part), using m_orgStaff makes the intent clearer: we're accessing the undo stack of the score containing the staff being modified, not the temporary UI copy used for staging changes.

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[RomanPudashkin]

@zacjansheski please test the "Staff/Part properties" dialog (there are no crashes or other problems when pressing Apply)