System markings: Re-enable Add > System marking after deleting one

[avvvvve]

Issue type

UX/Interaction bug (incorrect behaviour)

Description with steps to reproduce

This PR added handling that disables the Layout > Add > "System marking..." option when there are as many system markings as there are instruments in the score.

However, when you delete a system marking, the button does not become re-enabled immediately until you click another instrument/system marking row in the Layout list.

It should be re-enabled immediately

Supporting files, videos and screenshots

Screen.Recording.2025-02-21.at.5.30.43.PM.mov

What is the latest version of MuseScore Studio where this issue is present?

#26653 (Nightly build once this is merged)

Regression

No.

Operating system

macOS 15

Additional context

No response

Checklist

• This report follows the guidelines for reporting bugs and issues
• I have verified that this issue has not been logged before, by searching the issue tracker for similar issues
• I have attached all requested files and information to this report
• I have attempted to identify the root problem as concisely as possible, and have used minimal reproducible examples where possible