Add Remove Background feature using AI (rembg)

Frontend:
- Added "Remove Background (AI)" to Image menu
- Created remove_background.js module with dialog options
- Added removeBackground method to API service

Backend:
- Added /tools/remove-background-base64 endpoint for miniPaint frontend
- Uses rembg library for AI-powered background removal

Features:
- Automatically detects main subject and removes background
- Option to create as new layer or replace current
- Enables transparency mode after removal
- Works with any image layer

frontend/src/js/config-menu.js

@@ -283,6 +283,14 @@ const menuDefinition = [
 				name: 'Histogram',
 				ellipsis: true,
 				target: 'image/histogram.histogram'
+			},
+			{
+				divider: true
+			},
+			{
+				name: 'Remove Background (AI)',
+				ellipsis: true,
+				target: 'image/remove_background.remove_background'
 			}
 		]
 	},