Fix AI Paint tool processing state and use full U2net model

- Fix brush_select isProcessing flag not resetting after first use
  (reset in on_leave() when switching tools)
- Add onnxruntime dependency for U2net background removal
- Use full U2net model (176MB) instead of lightweight for better quality

https://claude.ai/code/session_01CLedz6CanT9t46KBvng3vz

backend/requirements.txt

@@ -18,6 +18,9 @@ torch==2.1.2
 torchvision==0.16.2
 segment-anything @ git+https://github.com/facebookresearch/segment-anything.git
 
+# ONNX Runtime for U2Net background removal (works with numpy<2.0)
+onnxruntime==1.16.3
+
 # NOTE: rembg (background removal) disabled due to dependency conflicts
 # rembg>=2.0.70 requires:
 #   - scikit-image>=0.26.0 which requires numpy>=2.0