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.

tests/test_pack.py

@@ -29,7 +29,7 @@ class PackTest(unittest.TestCase):
             finally:
                 pack.DIST_DIR = original_dist
 
-            self.assertEqual(output_path.name, "SnapForm.v0.9.27.zip")
+            self.assertEqual(output_path.name, "SnapForm.v0.9.28.zip")
             with ZipFile(output_path) as archive:
                 names = set(archive.namelist())