Input by duration: input cursor visual refinements

Issue type

UI bug (incorrect info or interface appearance)

Description with steps to reproduce

Because the input cursor (the droplet on a line) is a single icon, the width of its vertical line doesn't exactly match the width of the tall vertical beat lines that it moves to. It also doesn't get perfectly centered between the other ticks.

When you move the cursor back and forth with the left/right arrow keys, you can see that the position of the vertical lines jump around a bit as a result (see video below).

Perhaps to avoid this, the cursor should not be a line and balloon glued together as one element, but rather leave the original line as-is and superimpose the just balloon part on top of it. In which case we'd need another icon (or if it's easier for centering purposes, we could just supply the droplet SVG to use?)

Supporting files, videos and screenshots

https://github.com/user-attachments/assets/5e69cc48-492d-4049-9c9c-94c712bd8ad2

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

Nightly / https://github.com/musescore/MuseScore/pull/26367

Regression

No.

Operating system

All

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