feat: Add BEN2 background removal functionality and integrate into SnapForm

- Introduced BEN2 model for background removal in the panel and properties.
- Updated runtime to handle background removal commands and model downloads.
- Implemented background removal logic in worker, including image processing and saving results.
- Enhanced tests to cover new background removal features and ensure proper integration.
- Updated version to 0.9.28 to reflect new features and improvements.

src/snapform/properties.py

@@ -9,6 +9,11 @@ class SnapFormSettings(bpy.types.PropertyGroup):
         description="A single image or video file used for reconstruction",
         subtype="FILE_PATH",
     )
+    background_input_path: bpy.props.StringProperty(
+        name="Image",
+        description="A single image used to test BEN2 background removal",
+        subtype="FILE_PATH",
+    )
     video_frames: bpy.props.IntProperty(
         name="Video Frames",
         description="Maximum number of frames sampled evenly from a video",