fix: add polling interval to download progress loop

[RudreshRajvansh]

Description

The progress polling loop in BookDownloader.downloadBook had no delay, so it hit
DownloadManager and allocated a new Cursor every iteration for the whole download.
Measured ~198 queries/sec on a Pixel.

Adds a 250ms wait between polls. It sits at the bottom of the loop behind
if (!isDownloadFinished), so the first progress update still goes out right away and
finishing doesn't get held up an extra 250ms.

Same phone and build, both successful downloads:

before: 796 iterations in 4022ms (~198/sec)
after: 16 iterations in 4083ms (~3.9/sec)

Related Issue

Closes #305

Type of change

Bug fix, though it's really cleanup. Nothing users would notice.

Pull Request checklist

• The commit message uses the conventional commiting method.
• Made sure that your PR is not duplicate
• Tests: Nothing automated, the loop needs a real DownloadManager and an actual
  download. Checked it by hand with a counter in the loop, numbers are in [Bug]: Download progress loop has no delay, polls DownloadManager ~200x/sec #305.
• Screenshots: Nothing visual changed.

[starry-shivam]

Thanks for reporting this and for creating a PR with the fix.