fix(flutter): album/artist detail covers + cacheFirst tracing for hangs

Album detail screen rendered the cover slot as a bare slate Container
— no actual image. Wire ServerImage at /api/albums/<id>/cover (96dp,
rounded). Same for artist detail: ClipOval around ServerImage of the
artist's coverUrl, falling back to slate when empty.

Artist navigation hang: artistProvider goes through cacheFirst, which
had no logging — so we couldn't see whether the cold-cache fetch was
running. Add an optional `tag` parameter that, when set, debugPrints
each step (drift hit/miss, connectivity, fetch start/done/error).
Wire tags into artistProvider and artistAlbumsProvider, plus add the
3s connectivity timeout there too as belt-and-suspenders.

Next test pass should show cacheFirst[artist(<id>)]: lines that
pinpoint where the artist screen is sticking.

flutter_client/lib/library/library_providers.dart

@@ -61,8 +61,10 @@ final artistProvider =
     toResult: (rows) => rows.isEmpty
         ? const ArtistRef(id: '', name: '')
         : rows.first.toRef(),
-    isOnline: () async =>
-        (await ref.read(connectivityProvider.future)),
+    isOnline: () async => (await ref
+        .read(connectivityProvider.future)
+        .timeout(const Duration(seconds: 3), onTimeout: () => true)),
+    tag: 'artist($id)',
   );
 });
 
@@ -90,8 +92,10 @@ final artistAlbumsProvider =
       final artist = r.readTableOrNull(db.cachedArtists);
       return album.toRef(artistName: artist?.name ?? '');
     }).toList(),
-    isOnline: () async =>
-        (await ref.read(connectivityProvider.future)),
+    isOnline: () async => (await ref
+        .read(connectivityProvider.future)
+        .timeout(const Duration(seconds: 3), onTimeout: () => true)),
+    tag: 'artistAlbums($artistId)',
   );
 });