Downloads: show artwork for queued/preparing rows (fetch the poster first) #412

Seen on 1.8.0 (build 1523), iPhone. On the Offline screen, rows that are waiting for a download slot or preparing on the server show only the generic download-arrow placeholder tile, not the title's poster. A list of in-progress downloads is then hard to scan.

Cause. OfflineLibrarySnapshotBuilder (around line 40) builds row artwork only from record.posterURL, the locally cached side-asset poster. OfflinePosterTile (OfflineLibraryView.swift around line 594) falls back to the placeholder glyph when there is no artwork. The poster is fetched by DownloadSideAssetService with the other side assets, so it isn't on disk while a row waits for admission or server preparation.

Proposed fix. Fetch the poster first, as soon as the row is created (enqueue), independent of P4 admission. It is small, and it must stay within the existing side-asset ownership, generation and storage-accounting rules. Optionally, the tile could fall back to the online artwork through the shared artwork pipeline while the local copy is missing, then switch to the local file.

The placeholder glyph remains for rows with no artwork at all.

Acceptance: queue several downloads with the slot limit full. Every waiting or preparing row shows its poster within a moment of being queued, the poster survives relaunch, and storage accounting still counts it.