Fix U2Net, alertify dialogs, and improve AI Paint workflow

- Switch U2Net from onnxruntime to OpenCV DNN to avoid
  "cannot enable executable stack" error in Docker
- Add alertify dialog styling to fix white text on white
  background issue in popups
- Add offerFloatSelection() to AI Paint that prompts user
  after selection to move/scale it (Canva-like workflow)
- Auto-switch to Select tool after floating selection

https://claude.ai/code/session_01CLedz6CanT9t46KBvng3vz

backend/requirements.txt

@@ -18,8 +18,8 @@ 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: Using OpenCV DNN instead of onnxruntime for U2Net
+# (onnxruntime has executable stack issues in some Docker environments)
 
 # NOTE: rembg (background removal) disabled due to dependency conflicts
 # rembg>=2.0.70 requires: