#2790 Use aria2c for model downloads

Beschreibung

## Summary

Adds optional aria2c support for model downloads.

If aria2c is installed, setup uses it with multiple connections and resume support. If aria2c is not installed, setup falls back to the existing downloader unchanged.

## What changed

- Detect aria2c with `shutil.which("aria2c")`
- Use aria2c for HTTP(S) model downloads when available
- Preserve `.part` resume behavior
- Fall back transparently to the existing downloader if aria2c is unavailable or fails

## Extra Notes

Tested:
- fresh aria2c download
- interrupted aria2c download resumes correctly
- aria2c absent -> built-in downloader works
- interrupted built-in download resumes correctly
- aria2c restored -> resumes the same `.part` file

On my connection, large model downloads improved from roughly 70 MB/s to 110 MB/s.