Fix: cancel download not working

[mduccc]

Problem

Cancelling a download via CancelToken had no effect — the download continued running even after cancellation was requested.

Root Cause

In SmartDownloader.downloadWithProgress(), the .then() callback was not async and _downloadWithSmartRetry() was not awaited:

// Before (broken)
_ensureConfigured(foreground).then((_) {
  _downloadWithSmartRetry(...);  // unawaited!
}).whenComplete(() {
  cancellationListener?.cancel();  // fires immediately!
});