Fix resumed download average speed

[ishaanlabs-gg]

Fixes #1516.

When resuming a download, the progress task starts at the existing byte offset. The completion summary was using that cumulative completed byte count for both:

• the displayed total downloaded amount, and
• the numerator for average speed.

That made resumed downloads report speed as if the already-present bytes were downloaded during the current invocation.

This change keeps the displayed total size cumulative, but subtracts the starting offset when calculating average speed. It applies to both the progress-bar display and the status display.

Validation:

• First verified the new regression failed before the fix with 2.5 bytes/s for a resumed 2-byte transfer over 2 seconds.
• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 /Volumes/STOREJET/2/Dependencies/httpie/venv/bin/python -m pytest tests/test_downloads.py::TestDownloads::test_resumed_download_summary_uses_current_transfer_for_speed -q
• git diff --check

Note:

• I installed the package and minimal focused test dependencies in /Volumes/STOREJET/2/Dependencies/httpie/venv.
• Installing the full .[test] extra is currently blocked locally by an old transitive flasgger wheel build failure through pytest-httpbin/httpbin, so I could not run the httpbin-backed download tests in this environment.