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.