- Use availableGeometry() instead of full screen geometry for fullscreen mode
- This accounts for taskbars, panels, and other system UI elements that may overlap
- Set window geometry to available screen space before calling showFullScreen()
- Ensures all GUI content remains visible in fullscreen mode
- Resolves issue where bottom part of fullscreen window was hidden/cut off

This fixes the fullscreen display issue where the window extended beyond
the visible screen area, making the bottom content inaccessible.