fix: collab sync no longer overwrites layers with empty array

- Root cause: App.tsx collab sync effect called setLayers(collab.layers)
  even when collab.layers was empty, wiping out initialLayers
- Effect: RenderEngine.render() iterates layers to draw elements;
  with 0 layers, no elements render (appear to disappear)
- Fix: guard against empty collab.layers

Fixes: drawing tools work on desktop and mobile, objects persist after release

frontend/src/App.tsx

@@ -299,9 +299,12 @@ const CADEditor: React.FC<CADEditorProps> = ({ projectId, token, onNavigateBack
       setElements(merged);
     }
 
+    // Never overwrite local layers with empty remote layers (prevents losing initialLayers)
+    if (collab.layers.length > 0) {
       const sameLayers = collab.layers.length === layers.length &&
         collab.layers.every((l, i) => l.id === layers[i]?.id);
       if (!sameLayers) setLayers(collab.layers);
+    }
 
     const sameBlocks = collab.blocks.length === blocks.length &&
       collab.blocks.every((b, i) => b.id === blocks[i]?.id);