Cadify advertises STL, OBJ, GLB, PLY and 3MF viewing and conversion, with the file staying in the browser. That could be useful for a quick private check of a prototype.

The developer is clear that STL-to-STEP cannot recreate the original CAD feature history. Treat any claimed privacy and format compatibility as things to verify on a disposable file first.

What to do: Try a sample STL-to-3MF conversion and inspect it in Bambu Studio.