feat(flutter): Discover suggestions feed — web parity

The Flutter Discover screen was Lidarr-search-only; its empty state
showed a "Type to search" placeholder while web's /discover renders
the LB-derived out-of-library artist SuggestionFeed as its default
surface. Parity gap that slipped #356.

- ArtistSuggestion/SeedContribution model mirroring web types, with
  attributionText() matching web's "Because you liked/played X[, Y, and
  Z]." (Oxford comma, max 3).
- DiscoverApi.listSuggestions() → GET /api/discover/suggestions
  (image_url already resolved server-side from Lidarr, a7bea43).
- discover_screen: empty search box → suggestions feed (artist art +
  name + attribution + Request, reusing the existing createRequest +
  mutation-queue-replay flow with optimistic hide); typing → Lidarr
  search replaces; clearing → suggestions return. Mirrors web exactly.

Flutter-only; server endpoint unchanged.

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

flutter_client/lib/api/endpoints/discover.dart

@@ -1,11 +1,25 @@
 import 'package:dio/dio.dart';
 
+import '../../models/artist_suggestion.dart';
 import '../../models/lidarr.dart';
 
 class DiscoverApi {
   DiscoverApi(this._dio);
   final Dio _dio;
 
+  /// GET /api/discover/suggestions — out-of-library artist suggestions
+  /// (ListenBrainz-derived; image_url resolved on-demand from Lidarr,
+  /// may be empty). The server already filters in-library and
+  /// non-terminal-request candidates.
+  Future<List<ArtistSuggestion>> listSuggestions() async {
+    final r = await _dio.get<List<dynamic>>('/api/discover/suggestions');
+    final raw = r.data ?? const [];
+    return raw
+        .map((e) =>
+            ArtistSuggestion.fromJson((e as Map).cast<String, dynamic>()))
+        .toList(growable: false);
+  }
+
   /// GET /api/lidarr/search?q=...&kind=artist|album|track. Server has a
   /// 60s LRU cache for repeat queries so re-typing the same string in
   /// quick succession is cheap.

flutter_client/lib/discover/discover_screen.dart

@@ -8,6 +8,7 @@ import '../api/endpoints/discover.dart';
 import '../api/errors.dart';
 import '../cache/mutation_queue.dart';
 import '../library/library_providers.dart' show dioProvider;
+import '../models/artist_suggestion.dart';
 import '../models/lidarr.dart';
 import '../shared/widgets/main_app_bar_actions.dart';
 import '../theme/theme_extension.dart';
@@ -27,6 +28,22 @@ class _DiscoverScreenState extends ConsumerState<DiscoverScreen> {
   final _ctrl = TextEditingController();
   LidarrRequestKind _kind = LidarrRequestKind.artist;
   Future<List<LidarrSearchResult>>? _resultsFuture;
+  // Default (empty-search) surface: LB-derived out-of-library artist
+  // suggestions, mirroring web's SuggestionFeed.
+  Future<List<ArtistSuggestion>>? _suggestionsFuture;
+  final _requested = <String>{};
+
+  @override
+  void initState() {
+    super.initState();
+    _loadSuggestions();
+    // Clearing the box returns to suggestions (web swaps live too).
+    _ctrl.addListener(() {
+      if (_ctrl.text.trim().isEmpty && _resultsFuture != null) {
+        setState(() => _resultsFuture = null);
+      }
+    });
+  }
 
   @override
   void dispose() {
@@ -34,6 +51,55 @@ class _DiscoverScreenState extends ConsumerState<DiscoverScreen> {
     super.dispose();
   }
 
+  // Assigns the future only; callers trigger the rebuild (initState
+  // runs before first build, so setState here would be a no-op/warn).
+  void _loadSuggestions() {
+    _suggestionsFuture = ref
+        .read(_discoverApiProvider.future)
+        .then((api) => api.listSuggestions());
+  }
+
+  Future<void> _requestSuggestion(ArtistSuggestion s) async {
+    final fs = Theme.of(context).extension<FabledSwordTheme>()!;
+    final args = {
+      'kind': LidarrRequestKind.artist.wire,
+      'artistMbid': s.mbid,
+      'artistName': s.name,
+      'albumMbid': null,
+      'albumTitle': null,
+    };
+    try {
+      final api = await ref.read(_discoverApiProvider.future);
+      await api.createRequest(
+        kind: LidarrRequestKind.artist,
+        artistMbid: s.mbid,
+        artistName: s.name,
+      );
+      if (mounted) {
+        // Reassign the future first; the setState below rebuilds and
+        // the FutureBuilder picks up the fresh fetch (server now
+        // filters this candidate out). _requested hides it meanwhile.
+        _loadSuggestions();
+        setState(() => _requested.add(s.mbid));
+        ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+          content: Text('Requested: ${s.name}'),
+          backgroundColor: fs.iron,
+        ));
+      }
+    } on DioException catch (_) {
+      await ref
+          .read(mutationQueueProvider)
+          .enqueue(MutationKinds.requestCreate, args);
+      if (mounted) {
+        setState(() => _requested.add(s.mbid));
+        ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+          content: Text('Request queued: ${s.name}'),
+          backgroundColor: fs.iron,
+        ));
+      }
+    }
+  }
+
   void _runSearch() {
     final q = _ctrl.text.trim();
     if (q.isEmpty) {
@@ -153,13 +219,7 @@ class _DiscoverScreenState extends ConsumerState<DiscoverScreen> {
         ),
         Expanded(
           child: _resultsFuture == null
-              ? Center(
+              ? _buildSuggestions(fs)
-                  child: Text(
-                    'Type to search, then tap Request to send to Lidarr.',
-                    textAlign: TextAlign.center,
-                    style: TextStyle(color: fs.ash),
-                  ),
-                )
               : FutureBuilder<List<LidarrSearchResult>>(
                   future: _resultsFuture,
                   builder: (ctx, snap) {
@@ -199,6 +259,62 @@ class _DiscoverScreenState extends ConsumerState<DiscoverScreen> {
       ]),
     );
   }
+
+  Widget _buildSuggestions(FabledSwordTheme fs) {
+    return FutureBuilder<List<ArtistSuggestion>>(
+      future: _suggestionsFuture,
+      builder: (ctx, snap) {
+        if (snap.connectionState != ConnectionState.done) {
+          return const Center(child: CircularProgressIndicator());
+        }
+        final items = (snap.data ?? const <ArtistSuggestion>[])
+            .where((s) => !_requested.contains(s.mbid))
+            .toList(growable: false);
+        return ListView(
+          padding: const EdgeInsets.only(bottom: 16),
+          children: [
+            Padding(
+              padding: const EdgeInsets.fromLTRB(16, 12, 16, 4),
+              child: Column(
+                crossAxisAlignment: CrossAxisAlignment.start,
+                children: [
+                  Text('Suggested for you',
+                      style: TextStyle(
+                          color: fs.parchment,
+                          fontSize: 20,
+                          fontWeight: FontWeight.w500)),
+                  const SizedBox(height: 2),
+                  Text(
+                    "Out-of-library artists drawn from what you've liked and played.",
+                    style: TextStyle(color: fs.ash, fontSize: 13),
+                  ),
+                ],
+              ),
+            ),
+            if (snap.hasError)
+              Padding(
+                padding: const EdgeInsets.all(16),
+                child: Text("Couldn't load suggestions.",
+                    style: TextStyle(color: fs.ash)),
+              )
+            else if (items.isEmpty)
+              Padding(
+                padding: const EdgeInsets.all(16),
+                child: Text(
+                  'Listen to something or like an artist to start getting suggestions.',
+                  style: TextStyle(color: fs.ash),
+                ),
+              )
+            else
+              ...items.map((s) => _SuggestionTile(
+                    s: s,
+                    onRequest: () => _requestSuggestion(s),
+                  )),
+          ],
+        );
+      },
+    );
+  }
 }
 
 class _ResultTile extends StatelessWidget {
@@ -284,3 +400,63 @@ class _Pill extends StatelessWidget {
     );
   }
 }
+
+class _SuggestionTile extends StatelessWidget {
+  const _SuggestionTile({required this.s, required this.onRequest});
+  final ArtistSuggestion s;
+  final VoidCallback onRequest;
+
+  @override
+  Widget build(BuildContext context) {
+    final fs = Theme.of(context).extension<FabledSwordTheme>()!;
+    return Padding(
+      padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 10),
+      child: Row(children: [
+        ClipRRect(
+          borderRadius: BorderRadius.circular(6),
+          child: Container(
+            width: 56,
+            height: 56,
+            color: fs.slate,
+            child: s.imageUrl.isEmpty
+                ? Icon(Icons.person, color: fs.ash)
+                : CachedNetworkImage(
+                    imageUrl: s.imageUrl,
+                    fit: BoxFit.cover,
+                    fadeInDuration: const Duration(milliseconds: 120),
+                    fadeOutDuration: Duration.zero,
+                    errorWidget: (_, __, ___) =>
+                        Icon(Icons.person, color: fs.ash),
+                  ),
+          ),
+        ),
+        const SizedBox(width: 12),
+        Expanded(
+          child: Column(
+            crossAxisAlignment: CrossAxisAlignment.start,
+            children: [
+              Text(s.name,
+                  maxLines: 1,
+                  overflow: TextOverflow.ellipsis,
+                  style: TextStyle(color: fs.parchment, fontSize: 14)),
+              if (s.attributionText.isNotEmpty)
+                Text(s.attributionText,
+                    maxLines: 2,
+                    overflow: TextOverflow.ellipsis,
+                    style: TextStyle(color: fs.ash, fontSize: 12)),
+            ],
+          ),
+        ),
+        const SizedBox(width: 8),
+        FilledButton(
+          onPressed: onRequest,
+          style: FilledButton.styleFrom(
+            backgroundColor: fs.accent,
+            foregroundColor: fs.parchment,
+          ),
+          child: const Text('Request'),
+        ),
+      ]),
+    );
+  }
+}

flutter_client/lib/models/artist_suggestion.dart

@@ -0,0 +1,51 @@
+/// Mirrors web/src/lib/api/types.ts ArtistSuggestion / SeedContribution —
+/// one out-of-library artist from GET /api/discover/suggestions. image_url
+/// is resolved on-demand from Lidarr server-side (may be empty).
+class SeedContribution {
+  const SeedContribution({required this.name, required this.isLiked});
+
+  final String name;
+  final bool isLiked;
+
+  factory SeedContribution.fromJson(Map<String, dynamic> j) => SeedContribution(
+        name: j['name'] as String? ?? '',
+        isLiked: j['is_liked'] as bool? ?? false,
+      );
+}
+
+class ArtistSuggestion {
+  const ArtistSuggestion({
+    required this.mbid,
+    required this.name,
+    required this.imageUrl,
+    required this.attribution,
+  });
+
+  final String mbid;
+  final String name;
+  final String imageUrl;
+  final List<SeedContribution> attribution;
+
+  factory ArtistSuggestion.fromJson(Map<String, dynamic> j) => ArtistSuggestion(
+        mbid: j['mbid'] as String? ?? '',
+        name: j['name'] as String? ?? '',
+        imageUrl: j['image_url'] as String? ?? '',
+        attribution: ((j['attribution'] as List?) ?? const [])
+            .map((e) =>
+                SeedContribution.fromJson((e as Map).cast<String, dynamic>()))
+            .toList(growable: false),
+      );
+
+  /// Mirrors web SuggestionFeed.attributionText (Oxford comma, max 3).
+  String get attributionText {
+    if (attribution.isEmpty) return '';
+    final phrases = attribution
+        .map((s) => '${s.isLiked ? 'liked' : 'played'} ${s.name}')
+        .toList(growable: false);
+    if (phrases.length == 1) return 'Because you ${phrases[0]}.';
+    if (phrases.length == 2) {
+      return 'Because you ${phrases[0]} and ${phrases[1]}.';
+    }
+    return 'Because you ${phrases[0]}, ${phrases[1]}, and ${phrases[2]}.';
+  }
+}