Download episode art along with audio

Sorry for the cross-posting. I originally put this under feature requests in Discussions, but it seems like it's not getting much attention there. Plus, I think this is better characterized under "Polish".

**Problem:**
When a podcast is automatically downloaded, Pocket Casts does not download the episode art. Instead, it uses a placeholder image.

**Expected Behavior:**
I should be able to view the episode art offline without having to manually open the episode and load the artwork.

No files, tests, or entry points are named. Start by locating the automatic episode-download flow and the existing artwork loading and offline-storage handling; done means automatically downloaded episodes display their episode art offline instead of a placeholder.

Written by the indexing model from the issue text.