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/artist_detail_screen.dart

@@ -6,6 +6,7 @@ import '../api/endpoints/likes.dart';
 import '../likes/like_button.dart';
 import '../models/album.dart';
 import '../player/player_provider.dart';
+import '../shared/widgets/server_image.dart';
 import '../theme/theme_extension.dart';
 import 'library_providers.dart';
 import 'widgets/album_card.dart';
@@ -30,9 +31,18 @@ class ArtistDetailScreen extends ConsumerWidget {
           Padding(
             padding: const EdgeInsets.all(16),
             child: Row(children: [
-              Container(
-                width: 96, height: 96,
-                decoration: BoxDecoration(color: fs.slate, shape: BoxShape.circle),
+              ClipOval(
+                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),
+                        ),
+                ),
               ),
               const SizedBox(width: 16),
               Expanded(