Phase 4: Background Removal (Transformers.js)

- Added @xenova/transformers dependency
- useBackgroundRemoval hook with RMBG-1.4 model
- Client-side background removal with progress indicator
- Background removal button in properties panel (image elements only)
- ~170MB model cached after first download

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/package.json

@@ -14,7 +14,8 @@
     "react-dom": "^19.2.5",
     "react-konva": "^18.2.10",
     "konva": "^9.3.18",
-    "use-image": "^1.1.1"
+    "use-image": "^1.1.1",
+    "@xenova/transformers": "^2.17.2"
   },
   "devDependencies": {
     "@eslint/js": "^9.39.4",