fix(flutter): album/artist covers in artist detail + tighter grid

Drift cache intentionally drops cover_url (server-derived). The
adapters comment says "REST cold-cache fallback briefly shows the real
values before drift takes over" — but once drift takes over, covers
are empty forever. Fix per source:

- Album cover: server constructs the URL deterministically as
  /api/albums/<id>/cover (internal/api/convert.go:69). Mirror that
  in CachedAlbumAdapter.toRef so AlbumRef.coverUrl is non-empty
  whether the row came from a fresh fetch or a drift hit. Restores
  cover art on the artist detail album grid (and any other surface
  reading albums from drift).

- Artist cover: server picks a representative album and reuses its
  cover (convert.go:98). Drift doesn't store the pointer, so derive
  client-side via the artist's first loaded album. New _ArtistAvatar
  prefers a non-empty server-emitted coverUrl and falls back to
  /api/albums/<firstAlbumId>/cover, then slate while the album list
  is still loading.

Album grid spacing was off because childAspectRatio: 0.8 inflated
each cell taller than the AlbumCard's actual ~160dp footprint,
leaving a visible gap below every card. Switch to mainAxisExtent: 168
with explicit 8dp main/cross spacing — cells now match the card and
sit on a clean grid.

flutter_client/lib/cache/adapters.dart

@@ -34,12 +34,18 @@ extension ArtistRefDriftWrite on ArtistRef {
 
 extension CachedAlbumAdapter on CachedAlbum {
   /// `artistName` is supplied by the joined CachedArtists row at query time.
+  /// `coverUrl` is reconstructed deterministically from the album id —
+  /// the server emits the same shape (see internal/api/convert.go:69).
+  /// We don't need to persist it, so AlbumRef.coverUrl is non-empty
+  /// even when the row was populated from a sync that didn't carry the
+  /// derived URL.
   AlbumRef toRef({String artistName = ''}) => AlbumRef(
         id: id,
         title: title,
         sortTitle: sortTitle,
         artistId: artistId,
         artistName: artistName,
+        coverUrl: '/api/albums/$id/cover',
       );
 }