feat: show nix copy transfer stats

[elikoga]

Summary

• expose Nix FILE_TRANSFER byte progress alongside overall process progress
• persist transfer metrics through task status updates
• display transferred bytes, progress, active transfers, and failures in task details

Verification

• uv run pytest tests/nix/test_log_parse.py
• npm run build
• pre-commit hooks passed

The full frontend npm run check still reports unrelated existing diagnostics outside the changed files.