Save converted files to localStorage if not STEP or glTF, show in UI if conversions are locally available

[franknoirot]

Our API doesn't save converted files which makes sense, but users may expect them to be around later if they return to a generation, and not be charged anew to convert again. We can mitigate this by saving additional converted models to the user's localStorage, and if they are found on future revisits, showing the user that they won't need to be fetched again and just immediately downloading the stored version.