Preserve generated 3D model when switching between 2D and 3D modes

After generating a 3D model, switching from 3D mode back to 2D mode and then returning to 3D mode discards the generated model and requires it to be regenerated.

Expected behavior:
The generated 3D model should remain available when moving between 2D and 3D modes, as long as the underlying image/settings that affect the model have not changed.

Actual behavior:
Returning to 3D mode requires regenerating the model, even if the user only switched views.

Why this matters:
Regenerating the model can take time, especially for larger or more complex images. Losing the preview during normal navigation makes the workflow feel slower and can interrupt comparison between the 2D image and the generated 3D result.

Desired outcome:
Users should be able to move between 2D and 3D mode without losing the current generated model unless they make a change that actually requires regeneration.

Reproduce the issue by generating a 3D model, switching to 2D, and returning to 3D; then trace the mode-switching and model-generation entry points. Done means the existing model remains available when only the view changes, while changes to image or model-affecting settings still require regeneration.