Restore scroll position when switching between documents

[arturhg]

Summary

• Fixed scroll position not being restored when switching between documents during a session
• The issue was that the scroll value read from the scrollbar was unreliable after zoom operations modified it
• Fixed by storing the scroll value directly when loading from the edit file, and reading from that stored value instead of the scrollbar

Root Cause

When opening a document:

1. loadEdition() reads lastScrollValue from YAML and sets vScrollBar.setValue()
2. The subsequent zoom adjustment overwrites the scrollbar value
3. Original code read vScrollBar.getValue() after the zoom adjustment, getting the wrong value

Solution

• Added loadedScrollValue field in Edition.java to store the scroll value when loaded from file
• Read from that stored value instead of the scrollbar in MainScreen.java
• Apply with Platform.runLater() to ensure UI is ready

Test done

• Open a PDF, add annotation, scroll to a position
• Switch to another PDF
• Switch back - scroll position is now restored correctly

Closes #215