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

frontend/src/App.css

@@ -118,7 +118,10 @@
 .right-panel {
   display: flex;
   flex-direction: column;
-  gap: 20px;
+  gap: 12px;
+  overflow-y: auto;
+  max-height: calc(100vh - 180px);
+  padding-right: 8px;
 }
 
 .history-wrapper {