feat(flutter/models): User, ArtistRef, AlbumRef, TrackRef, HomeData

Field names mirror web/src/lib/api/types.ts and the server JSON shape
(internal/api/types.go's HomePayload). Hand-written DTOs per spec
section 10 -- codegen revisits at ~30 models.

Adopts server contract over plan-body mock:
- cover_url (not cover_art_url) for artist/album refs
- duration_sec (not duration_ms) for albums and tracks
- stream_url, disc_number on TrackRef
- sort_name/album_count on ArtistRef; sort_title/year/track_count on AlbumRef

HomeData section keys match server: recently_added_albums,
rediscover_albums, rediscover_artists, most_played_tracks,
last_played_artists.

flutter_client/lib/models/artist.dart

@@ -0,0 +1,30 @@
+// Mirrors internal/api/types.go ArtistRef and web/src/lib/api/types.ts.
+//
+// `cover_url` is the server's field name (NOT cover_art_url). Server emits
+// "" when the artist has no representative album cover, so we keep it
+// non-null here and let UI code branch on isEmpty. `sort_name` and
+// `album_count` are exposed by the server contract; we accept their
+// absence defensively (older fixtures, partial mocks).
+class ArtistRef {
+  const ArtistRef({
+    required this.id,
+    required this.name,
+    this.sortName = '',
+    this.albumCount = 0,
+    this.coverUrl = '',
+  });
+
+  final String id;
+  final String name;
+  final String sortName;
+  final int albumCount;
+  final String coverUrl;
+
+  factory ArtistRef.fromJson(Map<String, dynamic> j) => ArtistRef(
+        id: j['id'] as String,
+        name: j['name'] as String,
+        sortName: j['sort_name'] as String? ?? '',
+        albumCount: (j['album_count'] as num?)?.toInt() ?? 0,
+        coverUrl: j['cover_url'] as String? ?? '',
+      );
+}