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/cache/metadata_prefetcher.dart

@@ -5,11 +5,9 @@ import '../library/library_providers.dart';
 import '../models/home_data.dart';
 
 /// Pre-warms the drift cache for entities the user is most likely to
-/// tap. When /api/home returns its sections, fire one albumProvider /
-/// artistProvider read per top-of-section item in the background. The
-/// reads trigger the existing cold-cache path, which writes drift.
-/// By the time the user actually taps a tile, it's a drift hit and
-/// the detail screen renders instantly.
+/// tap. The home prefetch fires when /api/home returns. Library
+/// tabs and artist detail expose `warmAlbums` / `warmArtists` so they
+/// can fan out reads from their own AsyncNotifier callbacks.
 ///
 /// All reads are unawaited and idempotent — providers that already
 /// have a cached value short-circuit. Readers/writers don't conflict
@@ -28,6 +26,29 @@ class MetadataPrefetcher {
   /// the user can see without scrolling on a typical phone.
   static const _topN = 8;
 
+  /// Pre-warm drift for each album id (no-op past the first dedup).
+  /// Called from the library Albums tab + artist detail album grid.
+  void warmAlbums(Iterable<String> ids) {
+    var n = 0;
+    for (final id in ids) {
+      if (id.isEmpty) continue;
+      if (n++ >= _topN) break;
+      _swallow(_ref.read(albumProvider(id).future));
+    }
+    if (n > 0) debugPrint('metadataPrefetcher: warming $n albums');
+  }
+
+  /// Pre-warm drift for each artist id.
+  void warmArtists(Iterable<String> ids) {
+    var n = 0;
+    for (final id in ids) {
+      if (id.isEmpty) continue;
+      if (n++ >= _topN) break;
+      _swallow(_ref.read(artistProvider(id).future));
+    }
+    if (n > 0) debugPrint('metadataPrefetcher: warming $n artists');
+  }
+
   void _warmHome(HomeData h) {
     final albumIds = <String>{};
     final artistIds = <String>{};

flutter_client/lib/library/artist_detail_screen.dart

@@ -1,8 +1,10 @@
+import 'package:flutter/foundation.dart' show debugPrint;
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
 import '../api/endpoints/likes.dart';
+import '../cache/metadata_prefetcher.dart';
 import '../likes/like_button.dart';
 import '../models/album.dart';
 import '../models/artist.dart';
@@ -92,10 +94,33 @@ class ArtistDetailScreen extends ConsumerWidget {
                 child: IconButton(
                   icon: Icon(Icons.play_arrow, color: fs.parchment),
                   onPressed: () async {
-                    final tracks = await ref.read(artistTracksProvider(id).future);
-                    if (tracks.isEmpty) return;
-                    final shuffled = [...tracks]..shuffle();
-                    ref.read(playerActionsProvider).playTracks(shuffled);
+                    try {
+                      final tracks =
+                          await ref.read(artistTracksProvider(id).future);
+                      debugPrint(
+                          'artist_detail: play tapped — ${tracks.length} tracks for $id');
+                      if (tracks.isEmpty) {
+                        if (context.mounted) {
+                          ScaffoldMessenger.of(context).showSnackBar(
+                            const SnackBar(
+                                content: Text(
+                                    'No tracks found for this artist yet.')),
+                          );
+                        }
+                        return;
+                      }
+                      final shuffled = [...tracks]..shuffle();
+                      await ref
+                          .read(playerActionsProvider)
+                          .playTracks(shuffled);
+                    } catch (e) {
+                      debugPrint('artist_detail: play failed: $e');
+                      if (context.mounted) {
+                        ScaffoldMessenger.of(context).showSnackBar(
+                          SnackBar(content: Text("Couldn't start playback: $e")),
+                        );
+                      }
+                    }
                   },
                 ),
               ),
@@ -109,7 +134,11 @@ class ArtistDetailScreen extends ConsumerWidget {
           albums.when(
             error: (e, _) => Center(child: Text('$e', style: TextStyle(color: fs.error))),
             loading: () => const Padding(padding: EdgeInsets.all(16), child: Center(child: CircularProgressIndicator())),
-            data: (list) => LayoutBuilder(builder: (ctx, constraints) {
+            data: (list) {
+              ref
+                  .read(metadataPrefetcherProvider)
+                  .warmAlbums(list.map((a) => a.id));
+              return LayoutBuilder(builder: (ctx, constraints) {
               const cols = 3;
               const sidePad = 8.0;
               const gap = 8.0;
@@ -144,7 +173,8 @@ class ArtistDetailScreen extends ConsumerWidget {
                   );
                 },
               );
-            }),
+            });
+            },
           ),
         ]);
 }

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 {
                       ),
                     );
                   }),
-                ),
+                );
+          },
         );
   }
 }