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/track.dart

@@ -0,0 +1,43 @@
+// Mirrors internal/api/types.go TrackRef and web/src/lib/api/types.ts.
+//
+// Server uses `duration_sec` (NOT duration_ms). `track_number` and
+// `disc_number` are omitempty server-side so they're nullable here.
+// `stream_url` points at /api/tracks/{id}/stream.
+class TrackRef {
+  const TrackRef({
+    required this.id,
+    required this.title,
+    required this.albumId,
+    required this.artistId,
+    this.albumTitle = '',
+    this.artistName = '',
+    this.trackNumber,
+    this.discNumber,
+    this.durationSec = 0,
+    this.streamUrl = '',
+  });
+
+  final String id;
+  final String title;
+  final String albumId;
+  final String albumTitle;
+  final String artistId;
+  final String artistName;
+  final int? trackNumber;
+  final int? discNumber;
+  final int durationSec;
+  final String streamUrl;
+
+  factory TrackRef.fromJson(Map<String, dynamic> j) => TrackRef(
+        id: j['id'] as String,
+        title: j['title'] as String,
+        albumId: j['album_id'] as String,
+        albumTitle: j['album_title'] as String? ?? '',
+        artistId: j['artist_id'] as String,
+        artistName: j['artist_name'] as String? ?? '',
+        trackNumber: (j['track_number'] as num?)?.toInt(),
+        discNumber: (j['disc_number'] as num?)?.toInt(),
+        durationSec: (j['duration_sec'] as num?)?.toInt() ?? 0,
+        streamUrl: j['stream_url'] as String? ?? '',
+      );
+}