Resumed downloads overwrite partial data in non-interactive mode

[dajiaohuang]

What happened?\n\nWhen a partial download exists at the internal .tmp path, downloadFile sends a Range request starting at the existing size. In non-interactive mode it then copies the ranged response without seeking to the end of the already-open file, so the response starts at offset 0 and overwrites the partial bytes instead of appending them.\n\n### Version\n\nmain at d5d0b56\n\n### Relevant log output\n\nA hermetic HTTP-server reproduction is sufficient: seed a partial file, have the server return the remaining bytes for the requested Range, then call the download helper with no progress bar. The result contains the remainder at the beginning rather than the complete payload. Seeking before both copy paths preserves the intended resumed-download behavior.

[dajiaohuang]

I prepared a focused fix that seeks the partial file to its current end before either copy path writes the resumed response, with a hermetic regression covering non-interactive mode.

The repository currently limits opening pull requests to collaborators, so I could not submit it as a regular PR. The tested branch is available here:

main...dajiaohuang:ipatool:fix/536-resume-noninteractive-download

Validation completed on the branch:

• go test ./pkg/appstore: 6 passed
• go test ./...: 90 passed across 15 packages
• go build ./..., generation, formatting, and git diff --check passed

If this direction fits the project, a collaborator invitation would let me submit it through the repository's normal review process.