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.lock

@@ -351,7 +351,7 @@ packages:
     source: sdk
     version: "0.0.0"
   flutter_cache_manager:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: flutter_cache_manager
       sha256: "400b6592f16a4409a7f2bb929a9a7e38c72cceb8ffb99ee57bbf2cb2cecf8386"