feat(offline): #427 S4b — offline pools on Home beside system tiles

Operator's model: offline, surface the cache-backed pools right
where the (now play-disabled, S4a) system playlists sit, so Home
still has something to play.

- ShuffleSource gains recentlyPlayed() (cache by lastPlayedAt desc,
  liked included) and liked() (cache ∩ liked set); shared _refs()
  materializes ordered ids from cached_tracks/artists/albums.
  Shuffle-all reuses the same recency walk.
- Home Playlists row: when offlineProvider is true, prepend two
  tiles — "Recently played" / "Liked" — sized to PlaylistCard.
  Tap → shuffle+play that pool from cache; empty → snackbar.
  System tiles still render (play disabled per S4a) beside them.
- offline=false (online + all widget tests) → no extra tiles;
  placeholder-count tests unaffected.

Closes the S4 thread of the #427 umbrella (S4a + S4b).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/library/home_screen.dart

@@ -4,10 +4,13 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
 import '../api/errors.dart';
+import '../cache/offline_provider.dart';
+import '../cache/shuffle_source.dart';
 import '../cache/tile_providers.dart';
 import '../models/playlist.dart';
 import '../models/system_playlists_status.dart';
 import '../models/track.dart';
+import '../player/player_provider.dart';
 import '../playlists/playlists_provider.dart';
 import '../playlists/widgets/playlist_card.dart';
 import '../playlists/widgets/playlist_placeholder_card.dart';
@@ -63,6 +66,7 @@ class HomeScreen extends ConsumerWidget {
                 _PlaylistsSection(
                   playlists: allPlaylists.value?.owned ?? const [],
                   status: status.value ?? SystemPlaylistsStatus.empty(),
+                  offline: ref.watch(offlineProvider),
                 ),
                 _RecentlyAddedSection(ids: h.recentlyAddedAlbums),
                 _RediscoverSection(
@@ -222,23 +226,124 @@ class _CompactTrackSkeleton extends StatelessWidget {
 // ─── Sections ────────────────────────────────────────────────────────
 
 class _PlaylistsSection extends StatelessWidget {
-  const _PlaylistsSection({required this.playlists, required this.status});
+  const _PlaylistsSection({
+    required this.playlists,
+    required this.status,
+    required this.offline,
+  });
   final List<Playlist> playlists;
   final SystemPlaylistsStatus status;
+  final bool offline;
 
   @override
   Widget build(BuildContext context) {
     final items = _buildPlaylistsRow(playlists, status);
+    final children = <Widget>[
+      // Offline: surface the cache-backed pools where the (now
+      // play-disabled) system playlists sit, so there's something
+      // to play. #427 S4b.
+      if (offline) ...const [
+        _OfflinePoolCard(
+          label: 'Recently played',
+          icon: Icons.history,
+          kind: _OfflinePoolKind.recentlyPlayed,
+        ),
+        _OfflinePoolCard(
+          label: 'Liked',
+          icon: Icons.favorite,
+          kind: _OfflinePoolKind.liked,
+        ),
+      ],
+      for (final item in items)
+        if (item is _RealPlaylist)
+          PlaylistCard(playlist: item.playlist)
+        else
+          PlaylistPlaceholderCard(
+            label: (item as _PlaceholderPlaylist).label,
+            variant: item.variant,
+          ),
+    ];
     return HorizontalScrollRow(
       title: 'Playlists',
       height: 220,
-      children: items.map((item) {
-        if (item is _RealPlaylist) {
-          return PlaylistCard(playlist: item.playlist);
-        }
-        final ph = item as _PlaceholderPlaylist;
-        return PlaylistPlaceholderCard(label: ph.label, variant: ph.variant);
-      }).toList(),
+      children: children,
+    );
+  }
+}
+
+enum _OfflinePoolKind { recentlyPlayed, liked }
+
+/// Home tile for an offline cache-backed pool. Tapping shuffles +
+/// plays that pool from the local cache. Sized to match
+/// PlaylistCard so the row stays visually consistent.
+class _OfflinePoolCard extends ConsumerWidget {
+  const _OfflinePoolCard({
+    required this.label,
+    required this.icon,
+    required this.kind,
+  });
+  final String label;
+  final IconData icon;
+  final _OfflinePoolKind kind;
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final fs = Theme.of(context).extension<FabledSwordTheme>()!;
+    return SizedBox(
+      width: 176,
+      child: Material(
+        color: Colors.transparent,
+        child: InkWell(
+          onTap: () async {
+            final messenger = ScaffoldMessenger.of(context);
+            final src = ref.read(shuffleSourceProvider);
+            final refs = await switch (kind) {
+              _OfflinePoolKind.recentlyPlayed => src.recentlyPlayed(),
+              _OfflinePoolKind.liked => src.liked(),
+            };
+            if (refs.isEmpty) {
+              messenger.showSnackBar(
+                SnackBar(content: Text('No cached $label tracks yet')),
+              );
+              return;
+            }
+            await ref
+                .read(playerActionsProvider)
+                .playTracks(refs, shuffle: true);
+          },
+          child: Padding(
+            padding: const EdgeInsets.symmetric(horizontal: 8),
+            child: Column(
+              crossAxisAlignment: CrossAxisAlignment.start,
+              children: [
+                Container(
+                  width: 144,
+                  height: 144,
+                  decoration: BoxDecoration(
+                    color: fs.slate,
+                    borderRadius: BorderRadius.circular(6),
+                  ),
+                  child: Icon(icon, color: fs.accent, size: 56),
+                ),
+                const SizedBox(height: 8),
+                Text(
+                  label,
+                  maxLines: 1,
+                  overflow: TextOverflow.ellipsis,
+                  style: TextStyle(color: fs.parchment, fontSize: 14),
+                ),
+                Padding(
+                  padding: const EdgeInsets.only(top: 2),
+                  child: Text(
+                    'Offline',
+                    style: TextStyle(color: fs.ash, fontSize: 11),
+                  ),
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
     );
   }
 }