Fix #485, download Zhang files without requiring Content-Length

[sylvesterkaczmarek]

Fixes #485.

Use the streamed GET response for optional download-size metadata instead of requiring a HEAD Content-Length. Share the transport between the Zhang database and LUT downloads, tolerate absent/invalid sizes, and close network/progress resources on every exit.

Stage downloads beside the destination and publish only after the existing checksum matches. Interrupted, HTTP-failed, or checksum-failed downloads now leave existing files intact and remove their temporary file; checksum failures no longer leave an invalid database installed.

Validation

macOS arm64, Python 3.12.11:

• python -m unittest Tests.test_filing -v: 12 tests passed, exercising both public download functions.
• Includes a real localhost HTTP redirect followed by a chunked response without Content-Length, with real checksum verification.
• Failure cases cover HTTP/connection errors, interrupted transfers, checksum mismatch, output-file errors, and cancellation.
• Both public downloaders reproduced the original KeyError before the fix.
• Syntax/error checks, bytecode compilation, and git diff --check passed.

Add the offline suite to Application Testing before the existing sample-data tests. Full sensor processing, live multi-gigabyte downloads, and Windows/Linux execution were not run.

Contributor: Sylvester Kaczmarek, Personal. No third-party code added.