culling.text

1. [x] Add helper `cull_faces_by_normal` in app.py that deletes faces whose normal is close to the camera view axis (|n·[0,0,1]| < min_dot).
2. [x] Extend `generate_3d_model` with parameter `enable_face_culling=False`; before any mesh post-processing call the helper when enabled.
3. [x] Propagate the new flag through `create_3d_model` and `regenerate_3d_model` (add param, pass downstream).
4. [x] Insert a `gr.Checkbox` labelled "Enable Face Culling" in the UI; default True.
5. [x] Add checkbox to `generate_3d_button` and `regenerate_button` input lists.
6. [x] Keep backward compatibility (all new parameters have defaults so existing calls still work if checkbox omitted).
7. [ ] Manual test: run app, toggle checkbox on/off, verify spikes disappear when on and original behaviour when off.