fix: collab sync overwrites blocks with empty array

- collab.blocks sync had no guard against empty arrays
- when project loads, collab syncs empty blocks from server
- this overwrites createDefaultBlocks() with empty array
- added if (collab.blocks.length > 0) guard, same as layers

frontend/src/App.tsx

@@ -309,9 +309,12 @@ const CADEditor: React.FC<CADEditorProps> = ({ projectId, token, onNavigateBack
       if (!sameLayers) setLayers(collab.layers);
     }
 
-    const sameBlocks = collab.blocks.length === blocks.length &&
-      collab.blocks.every((b, i) => b.id === blocks[i]?.id);
-    if (!sameBlocks) setBlocks(collab.blocks);
+    // Never overwrite local blocks with empty remote blocks (prevents losing defaultBlocks)
+    if (collab.blocks.length > 0) {
+      const sameBlocks = collab.blocks.length === blocks.length &&
+        collab.blocks.every((b, i) => b.id === blocks[i]?.id);
+      if (!sameBlocks) setBlocks(collab.blocks);
+    }
   // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [collab.elements, collab.layers, collab.blocks, collab.status]);