feat(flutter): extend metadata prefetch to library tabs + artist detail

MetadataPrefetcher gains warmAlbums(ids) / warmArtists(ids) public
methods so callers can fan out drift-cache warm-ups for whatever
collection just landed.

Wired into:
- Library Artists tab — warms first 8 artists from the page on every
  data emit (initial + paginate + refresh).
- Library Albums tab — same for first 8 albums.
- Artist detail album grid — warms first 8 albums from the artist's
  album list as soon as it loads, so tapping into any of them is a
  drift hit.

Hard-cap of 8 per call (same as the home prefetch). Set spans across
calls aren't deduped at this layer because providers themselves
short-circuit on cached values.

Also instrument the artist-detail play button: try/catch around the
artistTracksProvider read + playTracks call, snackbar on
empty-tracks or thrown-error so silent failures stop being silent.
The current behavior was an early return on tracks.isEmpty with no
visible feedback.

flutter_client/lib/library/library_screen.dart

@@ -5,6 +5,7 @@ import 'package:go_router/go_router.dart';
 import '../api/endpoints/library_lists.dart';
 import '../api/endpoints/likes.dart';
 import '../api/endpoints/me.dart';
+import '../cache/metadata_prefetcher.dart';
 import '../library/library_providers.dart' show dioProvider;
 import '../models/album.dart';
 import '../models/artist.dart';
@@ -207,7 +208,12 @@ class _ArtistsTab extends ConsumerWidget {
     return ref.watch(_libraryArtistsProvider).when(
           loading: () => const Center(child: CircularProgressIndicator()),
           error: (e, _) => Center(child: Text('$e', style: TextStyle(color: fs.error))),
-          data: (page) => page.items.isEmpty
+          data: (page) {
+            // Warm details for the first screenful so taps are instant.
+            ref
+                .read(metadataPrefetcherProvider)
+                .warmArtists(page.items.map((a) => a.id));
+            return page.items.isEmpty
               ? Center(child: Text("No artists yet — scan a library folder via the server's config.", style: TextStyle(color: fs.ash), textAlign: TextAlign.center))
               : RefreshIndicator(
                   onRefresh: () async {
@@ -247,7 +253,8 @@ class _ArtistsTab extends ConsumerWidget {
                       },
                     ),
                   ),
-                ),
+                );
+          },
         );
   }
 }
@@ -261,7 +268,11 @@ class _AlbumsTab extends ConsumerWidget {
     return ref.watch(_libraryAlbumsProvider).when(
           loading: () => const Center(child: CircularProgressIndicator()),
           error: (e, _) => Center(child: Text('$e', style: TextStyle(color: fs.error))),
-          data: (page) => page.items.isEmpty
+          data: (page) {
+            ref
+                .read(metadataPrefetcherProvider)
+                .warmAlbums(page.items.map((a) => a.id));
+            return page.items.isEmpty
               ? Center(child: Text("No albums yet — scan a library folder via the server's config.", style: TextStyle(color: fs.ash), textAlign: TextAlign.center))
               : RefreshIndicator(
                   onRefresh: () async {
@@ -315,7 +326,8 @@ class _AlbumsTab extends ConsumerWidget {
                       ),
                     );
                   }),
-                ),
+                );
+          },
         );
   }
 }