Support easily associating .puffin files with Puffin Viewer

Maybe even suggest such file association and set it up on startup of puffin viewer. So it is easy to share and open files in puffin viewer. Would require platform specifics to set up such a file association though.

Should also default to that as an the extension when saving reports.

The issue names no files or tests; start at the Puffin Viewer startup path and the report-saving entry point. Trace how the viewer opens reports and how saved filenames are chosen, then inspect the platform-specific requirements for associating .puffin files and verify that opening and saving use the intended extension.

Written by the indexing model from the issue text.