Undefined region plotting

[viktormai]

Added support for identifying and visualizing internal undefined regions in geometry plots:

• Added an option to enable/disable undefined-region coloring in the GUI.
• Added configurable coloring for internal undefined regions through the color settings menu, analogous to overlaps.
• Used classification of undefined regions from model.py to distinguish internal enclosed regions from external background regions.
• Added hover/status information and context menu support for identified internal undefined regions.
• Added storage of undefined-region masks during plot generation for visualization and interaction.

Motivation:

• Provides a clearer debugging tool for locating geometry gaps or missing definitions in OpenMC models.
• Helps users distinguish problematic internal voids from expected external undefined space.