Integrate PixiCanvas into Editor, remove Fabric.js dependency

- Rewrite Editor.tsx: swap FabricCanvas for PixiCanvas, wire
  SelectionManager, SceneManager, and all Pixi-based APIs
- Port tools.ts from Fabric Canvas API to PixiJS Viewport/Scene
- Update LayerPanel type icons for PixiJS scene types (text, video)
- Remove FabricCanvas.tsx (dead code after swap)
- Uninstall fabric npm package
- All operations, shortcuts, sync, history, drag/drop, paste now
  flow through the PixiJS pipeline end-to-end

frontend/package.json

@@ -11,7 +11,6 @@
   "dependencies": {
     "@pixi/filter-drop-shadow": "^5.2.0",
     "axios": "^1.7.9",
-    "fabric": "^6.5.1",
     "pixi-viewport": "^6.0.3",
     "pixi.js": "^8.17.0",
     "react": "^18.3.1",