It gives you the ability to analyze, compare, split, or merge tcx, gpx, and fit files.

Nothing

This page is a combination of HTML, Javascript, CSS, and WebAssembly compiled from Rust.

Nothing. All processing is done in the browser. No data leaves the local machine.

Not right now, but maybe in the future.

Yes, it is available on GitHub.

Though this page is fairly self-explanatory, there is more documentation on the GitHub wiki page.

Safari has strict cross origin protections that prevent loading the map data from another domain.