Add advanced editing features: layers, background removal, smart selection

Backend:
- Add /tools router with background removal, smart select, color select
- Add rembg dependency for AI background removal
- Add layer management API (list, flatten)
- Fix transparency preservation in blend_patch (veil collapse fix)
- Preserve alpha channel when reverting/resetting images

Frontend:
- Add AdvancedTools panel with background removal, smart select, color select
- Add Layers panel with drag-to-reorder, visibility toggle, flatten
- Add toolsApi for new backend endpoints
- Make right panel scrollable for additional controls

This adds "Photoshop light" capabilities:
- Remove background and create layer
- Smart object selection (click to select)
- Color selection with tolerance
- Layer system with compositing

backend/requirements.txt

@@ -14,3 +14,5 @@ pydantic-settings==2.1.0
 email-validator==2.1.0
 opencv-python-headless==4.9.0.80
 scikit-image==0.22.0
+rembg==2.0.50
+onnxruntime==1.16.3