Make save_large_file robust to transient errors

[Sriram-Nambiar]

Replace wget with streaming requests and Range-based resume in save_large_file, fixing #348.

wget treats transient HTTP errors (502/503/504/429) and DNS failures as fatal, and its -t counter is a budget for the entire download rather than consecutive failures without progress.

This matches the solution settled in openzim/zimwright#7:

• Resumes downloads using HTTP Range requests from the bytes on disk.
• Only counts attempts that make no progress toward the retry limit.
• Enforces explicit connect (30s) and read (60s) timeouts to avoid streaming hangs.
• Retries transient HTTP errors (429, 500, 502, 503, 504) and connection/timeout errors with backoff and jitter.
• Restarts cleanly from scratch if the server ignores Range (200 OK).

Unit tests added covering all resume, retry, and failure scenarios.

Closes #348.

[benoit74]

@Sriram-Nambiar this is a duplicate of #349 ; I will hence close this PR in favor of the other one. I strongly recommend to comment on issue to be assigned before starting work, this avoids this kind of duplicate effort.

I encourage you to contribute to #349 by reviewing code changes over there.

Please also do not merge from main to feature branch, this is against our practices and just adds noise (if you really need a change done in main, then you have to rebase and force push ; if review is OK, then you rebase and force push ; otherwise, you keep the branch a bit outdated, this eases the review process)