Run image preprocessing on GPU

Decorate on_image_change with @spaces.GPU so BiRefNet runs on the GPU
worker instead of falling back to the CPU-only main process.

app.py

@@ -85,6 +85,7 @@ def _viewer_iframe(ply_path: Path) -> str:
 # Event handlers
 # ----------------------------------------------------------------------------
 
+@spaces.GPU
 def on_image_change(image):
     """Run preprocessing as soon as the input changes — gives the user instant
     feedback on the matte/crop without waiting for the full generation."""