v2.6.4 - Fix Download Completion

Fixes the update download getting stuck at 100%.

Root cause: DownloadWorker defined 'finished = Signal(bool)' which shadowed QThread.finished. The signal was never emitted, so _on_download_finished never ran.

Fix: Removed the shadowing signal definition. Now QThread.finished fires when run() returns, and _on_download_finished reads the stored result.