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/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]}.';
+  }
+}