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/src/components/LayerPanel.tsx

@@ -85,8 +85,9 @@ export default function LayerPanel({
   function getTypeIcon(type: string, isGroup: boolean) {
     if (isGroup) return '\u25B8'; // triangle
     if (type === 'image') return '\u{1F5BC}';
-    if (type === 'i-text') return 'T';
+    if (type === 'text' || type === 'i-text') return 'T';
     if (type === 'path') return '\u270E';
+    if (type === 'video') return '\u25B6';
     return '\u25C7';
   }