Clean up model workers after download and load errors

[vietanhdev]

Hardens the crash/hang path reported in #208 when an auto-label model fails after download. The issue does not contain enough model/version detail to reproduce a native runtime crash, so this PR fixes the concrete unhandled Python/Qt path without auto-closing the report.

• Put download, extraction, registry lookup, and initialization under one error boundary
• Guarantee GenericWorker.finished is emitted with finally
• Log background failures instead of letting them escape a Qt worker slot
• Return the model UI to its idle state after failure
• Add worker and corrupt-download regression tests

Validation:

• 135 local unit tests passed (13 model-download skips)
• Manual threaded corrupt-archive reproduction confirmed thread cleanup, UI completion, and error status
• Ruff and diff checks passed