Use resolve_dir for direct URL destination (follow-up to #19)

[bradleesand]

Summary

Follow-up to #19 addressing @loganbuilt's review comment:

• _run_direct_url_with_cli now calls resolve_dir((destination or "").strip(), paths.single_downloads_dir) instead of doing its own os.path.isabs / os.path.join dance — matches the queue path in engine/job_queue.py:_resolve_job_output_dir.
• Destinations that escape the base (e.g. ../etc) now raise ValueError instead of silently resolving outside DOWNLOADS_DIR.
• Adds two regression tests in tests/test_direct_url_contracts.py:
  • relative destination "Singles" lands under single_downloads_dir/Singles/
  • escape destination "../../etc" is rejected and download_with_ytdlp is never invoked

Known test-environment issue

On a fresh process under Python 3.11.15 the new tests (and the rest of test_direct_url_contracts.py) fail at fixture setup with:

ValueError: failed to parse CPython sys.version: '3.11.9'

This comes from platform._sys_version_parser choking on the bare "3.11.9" string installed by the existing monkeypatch.setattr(sys, "version", "3.11.9", ...) line in the api_module fixture, after wsgiref.simple_server is imported transitively via google_auth_oauthlib during api.main import. Whether it triggers depends on whether wsgiref.simple_server was already cached in sys.modules before the fixture runs. The same monkeypatch pattern is used in ~19 fixtures across tests/, so this isn't unique to this file.

Not touching the fixture in this PR to keep the diff focused on the review comment. Planning a follow-up PR to bring the full test suite under GitHub Actions; once CI is running the whole test_* family we can fix the fixture pattern once for everyone.

Separately, two pre-existing tests in this file (test_*_video_mode_*) fail on main with a final_format assertion mismatch ('mp3' != 'mkv'), unrelated to this change — left untouched.

Test plan

• pytest tests/test_direct_url_contracts.py::test_server_direct_url_relative_destination_resolves_under_downloads_dir — passes in environments where the fixture's sys.version patch doesn't break wsgiref import
• pytest tests/test_direct_url_contracts.py::test_server_direct_url_escape_destination_is_rejected — same caveat
• Manual: submit a direct URL with single_download_folder="Singles" → file in $DOWNLOADS_DIR/Singles/
• Manual: submit a direct URL with destination="../outside" → rejected with a clear error