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',
       );
 }
 

flutter_client/lib/library/artist_detail_screen.dart

@@ -35,13 +35,16 @@ class ArtistDetailScreen extends ConsumerWidget {
                 child: SizedBox(
                   width: 96,
                   height: 96,
-                  child: a.coverUrl.isEmpty
-                      ? Container(color: fs.slate)
-                      : ServerImage(
-                          url: a.coverUrl,
-                          fit: BoxFit.cover,
-                          fallback: Container(color: fs.slate),
-                        ),
+                  // Server derives artist cover from a representative
+                  // album. Drift cache doesn't persist that pointer, so
+                  // mirror the trick client-side: reuse the first album
+                  // returned by artistAlbumsProvider. Falls back to
+                  // slate while albums are loading or empty.
+                  child: _ArtistAvatar(
+                    serverCoverUrl: a.coverUrl,
+                    albums: albums,
+                    fs: fs,
+                  ),
                 ),
               ),
               const SizedBox(width: 16),
@@ -82,9 +85,16 @@ class ArtistDetailScreen extends ConsumerWidget {
               shrinkWrap: true,
               physics: const NeverScrollableScrollPhysics(),
               padding: const EdgeInsets.all(8),
+              // mainAxisExtent locks each cell's height to the actual
+              // AlbumCard footprint (124 cover + 8 + 14 title + 12
+              // artist + ~6 padding), instead of letting
+              // childAspectRatio inflate it. Removes the visible vertical
+              // gap below each card.
               gridDelegate: const SliverGridDelegateWithFixedCrossAxisCount(
                 crossAxisCount: 2,
-                childAspectRatio: 0.8,
+                mainAxisExtent: 168,
+                mainAxisSpacing: 8,
+                crossAxisSpacing: 8,
               ),
               itemCount: list.length,
               itemBuilder: (_, i) {
@@ -98,3 +108,37 @@ class ArtistDetailScreen extends ConsumerWidget {
     );
   }
 }
+
+/// Renders the artist's avatar. Server-emitted coverUrl wins when
+/// non-empty; otherwise we mirror the server's "use the first album's
+/// cover" rule client-side via the loaded album list.
+class _ArtistAvatar extends StatelessWidget {
+  const _ArtistAvatar({
+    required this.serverCoverUrl,
+    required this.albums,
+    required this.fs,
+  });
+  final String serverCoverUrl;
+  final AsyncValue<List<AlbumRef>> albums;
+  final FabledSwordTheme fs;
+
+  @override
+  Widget build(BuildContext context) {
+    if (serverCoverUrl.isNotEmpty) {
+      return ServerImage(
+        url: serverCoverUrl,
+        fit: BoxFit.cover,
+        fallback: Container(color: fs.slate),
+      );
+    }
+    final firstId = albums.value?.isNotEmpty == true ? albums.value!.first.id : null;
+    if (firstId == null) {
+      return Container(color: fs.slate);
+    }
+    return ServerImage(
+      url: '/api/albums/$firstId/cover',
+      fit: BoxFit.cover,
+      fallback: Container(color: fs.slate),
+    );
+  }
+}