fix(flutter): restore artist coverUrl on drift-cached ArtistRef

Multi-artist surfaces (home Rediscover, Library Artists tab, Liked
Artists carousel) were all rendering the music-notes placeholder
instead of real artist covers. Root cause:

CachedArtist.toRef() returned an ArtistRef with empty coverUrl —
the cache doesn't store the representative album id the server
derives at query time, and the adapter never reconstructed it.
The artist detail screen worked coincidentally because it derives
its header cover from `artist.albums[0].id` directly rather than
the ArtistRef's coverUrl field.

Fix:
* CachedArtistAdapter.toRef() now accepts an optional coverAlbumId
  and reconstructs `/api/albums/<id>/cover` when given. Matches the
  pattern AlbumRef uses (deterministic URL from entity id).
* artistTileProvider, libraryArtistsProvider, and artistProvider
  (single-artist) each LEFT JOIN cached_albums ordered by sort_title.
  First row per artist carries the alphabetically-first album id;
  toRef projects that into the cover URL.
* Multi-artist queries dedup in toResult since the join multiplies
  rows by album count.

Artists with no albums yet in drift come through with empty
coverUrl — UI falls back to the music-notes placeholder, same
behavior as before for that legitimately-coverless state.

flutter_client/lib/library/library_providers.dart

@@ -233,17 +233,28 @@ Map<String, dynamic> _trackToJson(TrackRef t) => {
 final artistProvider =
     StreamProvider.family<ArtistRef, String>((ref, id) {
   final db = ref.watch(appDbProvider);
-  return cacheFirst<CachedArtist, ArtistRef>(
-    driftStream: (db.select(db.cachedArtists)..where((t) => t.id.equals(id)))
-        .watch(),
+  // LEFT JOIN cached_albums for cover-URL reconstruction (see
+  // CachedArtistAdapter.toRef). First row carries the alphabetically-
+  // first album.
+  final query = (db.select(db.cachedArtists)..where((t) => t.id.equals(id)))
+      .join([
+    drift.leftOuterJoin(db.cachedAlbums,
+        db.cachedAlbums.artistId.equalsExp(db.cachedArtists.id)),
+  ])
+    ..orderBy([drift.OrderingTerm.asc(db.cachedAlbums.sortTitle)]);
+  return cacheFirst<drift.TypedResult, ArtistRef>(
+    driftStream: query.watch(),
     fetchAndPopulate: () async {
       final api = await ref.read(libraryApiProvider.future);
       final fresh = await api.getArtist(id);
       await db.into(db.cachedArtists).insertOnConflictUpdate(fresh.toDrift());
     },
-    toResult: (rows) => rows.isEmpty
-        ? const ArtistRef(id: '', name: '')
-        : rows.first.toRef(),
+    toResult: (rows) {
+      if (rows.isEmpty) return const ArtistRef(id: '', name: '');
+      final artist = rows.first.readTable(db.cachedArtists);
+      final firstAlbum = rows.first.readTableOrNull(db.cachedAlbums);
+      return artist.toRef(coverAlbumId: firstAlbum?.id ?? '');
+    },
     isOnline: () async => (await ref
         .read(connectivityProvider.future)
         .timeout(const Duration(seconds: 3), onTimeout: () => true)),

flutter_client/lib/library/library_screen.dart

@@ -62,9 +62,19 @@ final _meApiProvider = FutureProvider<MeApi>((ref) async {
 /// via /api/library/sync.
 final _libraryArtistsProvider = StreamProvider<List<ArtistRef>>((ref) {
   final db = ref.watch(appDbProvider);
-  final query = db.select(db.cachedArtists)
-    ..orderBy([(t) => drift.OrderingTerm.asc(t.sortName)]);
-  return cacheFirst<CachedArtist, List<ArtistRef>>(
+  // LEFT JOIN cached_albums so each artist row carries a representative
+  // album id for cover-URL reconstruction. Sorted by artist sortName
+  // then album sortTitle: the first row per artist gets the
+  // alphabetically-first album. Dedup happens in toResult.
+  final query = db.select(db.cachedArtists).join([
+    drift.leftOuterJoin(db.cachedAlbums,
+        db.cachedAlbums.artistId.equalsExp(db.cachedArtists.id)),
+  ])
+    ..orderBy([
+      drift.OrderingTerm.asc(db.cachedArtists.sortName),
+      drift.OrderingTerm.asc(db.cachedAlbums.sortTitle),
+    ]);
+  return cacheFirst<drift.TypedResult, List<ArtistRef>>(
     driftStream: query.watch(),
     fetchAndPopulate: () async {
       // Cold-cache fallback: sync should have populated drift already,
@@ -80,7 +90,23 @@ final _libraryArtistsProvider = StreamProvider<List<ArtistRef>>((ref) {
         );
       });
     },
-    toResult: (rows) => rows.map((r) => r.toRef()).toList(),
+    toResult: (rows) {
+      // The join multiplies rows by album count; dedup by artist id
+      // keeping the first occurrence so we get one ArtistRef per
+      // artist with the alphabetically-first album's id for the
+      // cover-URL projection. Artists with no albums in drift yet
+      // come through as a single row with null cachedAlbums and an
+      // empty coverUrl — UI falls back to the placeholder icon.
+      final seen = <String>{};
+      final out = <ArtistRef>[];
+      for (final r in rows) {
+        final a = r.readTable(db.cachedArtists);
+        if (!seen.add(a.id)) continue;
+        final album = r.readTableOrNull(db.cachedAlbums);
+        out.add(a.toRef(coverAlbumId: album?.id ?? ''));
+      }
+      return out;
+    },
     isOnline: () async => (await ref
         .read(connectivityProvider.future)
         .timeout(const Duration(seconds: 3), onTimeout: () => true)),