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/main.py

@@ -5,7 +5,7 @@ from contextlib import asynccontextmanager
 
 from app.config import settings
 from app.database import init_db
-from app.routers import projects, edits, images, patches, generate
+from app.routers import projects, edits, images, patches, generate, tools
 
 
 @asynccontextmanager
@@ -37,6 +37,7 @@ app.include_router(edits.router)
 app.include_router(images.router)
 app.include_router(patches.router)
 app.include_router(generate.router)
+app.include_router(tools.router)
 
 
 @app.get("/")