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/app/services/edit_service.py

@@ -188,9 +188,11 @@ class EditService:
         if not result_path.exists():
             raise FileNotFoundError(f"Edit {edit_id} result not found")
 
-        # Copy result to current
+        # Copy result to current (preserve alpha channel)
         current_path = self.get_current_image_path(project_id)
-        Image.open(result_path).save(current_path)
+        img = Image.open(result_path)
+        # Preserve original mode to maintain transparency
+        img.save(current_path, format='PNG')
 
         return str(current_path)
 
@@ -210,7 +212,9 @@ class EditService:
         if not original_path.exists():
             raise FileNotFoundError(f"Original image for project {project_id} not found")
 
-        # Copy original to current
-        Image.open(original_path).save(current_path)
+        # Copy original to current (preserve alpha channel)
+        img = Image.open(original_path)
+        # Preserve original mode to maintain transparency
+        img.save(current_path, format='PNG')
 
         return str(current_path)