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/album_detail_screen.dart

@@ -6,6 +6,7 @@ import '../cache/audio_cache_manager.dart';
 import '../cache/db.dart';
 import '../likes/like_button.dart';
 import '../player/player_provider.dart';
+import '../shared/widgets/server_image.dart';
 import '../theme/theme_extension.dart';
 import 'library_providers.dart';
 import 'widgets/track_row.dart';
@@ -27,7 +28,18 @@ class AlbumDetailScreen extends ConsumerWidget {
               Padding(
                 padding: const EdgeInsets.all(16),
                 child: Row(children: [
-                  Container(width: 96, height: 96, color: fs.slate),
+                  ClipRRect(
+                    borderRadius: BorderRadius.circular(6),
+                    child: SizedBox(
+                      width: 96,
+                      height: 96,
+                      child: ServerImage(
+                        url: '/api/albums/$id/cover',
+                        fit: BoxFit.cover,
+                        fallback: Container(color: fs.slate),
+                      ),
+                    ),
+                  ),
                   const SizedBox(width: 16),
                   Expanded(child: Column(
                     crossAxisAlignment: CrossAxisAlignment.start,