Fix bb review issues: stale snapshot, double-tap guard, button dedup, comments

- _downloadEpisodeAt: read _episodes fresh inside setState instead of using
  the pre-await snapshot, and guard that the index is still valid and the row
  still lacks a mediaId, preventing silent overwrites of data refreshed by
  _load() during the await.
- Add Set<int> _pendingDownloads to prevent concurrent download API calls when
  the user double-taps; visually disable the button while in-flight via new
  isLoading parameter on _DownloadButton.
- Extract _EpisodeActionButton shared primitive to eliminate structural
  duplication between _PlayButton and _DownloadButton (DRY).
- Fix misleading mediaId null-safety comment to describe the actual guarantee.
- Add comments to episodeToggleErrorMessage and episodeDownloadErrorMessage
  explaining why they use action-specific fallback strings.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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