Encapsulate shouldDisplayUndoBookmark with private setter

[Namyunsuk]

What I have done and why

This PR encapsulates the shouldDisplayUndoBookmark state in BookmarksViewModel by adding a private set.

Previously, shouldDisplayUndoBookmark was publicly mutable, which allowed external classes or composables to modify its value directly. By making the setter private, only the ViewModel can update this state. This change improves encapsulation and prevents unintended modifications from outside.

Fix #1899