feat(flutter): pre-warm covers during library sync

Slice 2 of the cover-caching pass. SyncController now downloads cover
bytes for newly-upserted albums + playlists into the shared
flutter_cache_manager disk cache after each sync transaction commits.
A cold-start scroll through the home grid paints from disk on the very
first frame instead of firing one HTTP per visible tile.

Best-effort: fire-and-forget after commit, concurrency 3, per-URL
failures swallowed (404 for collages that haven't built yet, 401
during token-refresh races). Artist covers skipped — ArtistRef.coverUrl
is server-derived from "most-recent album" and not reconstructible
client-side; album pre-warm already covers the artist's primary visual.

Auth header reuses sessionTokenProvider for parity with ServerImage.

flutter_client/pubspec.yaml

@@ -37,6 +37,12 @@ dependencies:
   # by ServerImage (auth-aware path) and as CachedNetworkImageProvider
   # for the mini bar (which composes its own Image widget).
   cached_network_image: ^3.4.1
+  # flutter_cache_manager is the disk-cache layer cached_network_image
+  # sits on top of. SyncController uses DefaultCacheManager directly to
+  # pre-warm covers during metadata sync, so a cold-start home grid
+  # paints from disk on the very first scroll rather than firing a
+  # network round-trip per tile.
+  flutter_cache_manager: ^3.4.1
 
 dev_dependencies:
   flutter_test: