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/album.dart

@@ -0,0 +1,40 @@
+// Mirrors internal/api/types.go AlbumRef and web/src/lib/api/types.ts.
+//
+// `cover_url` (NOT cover_art_url) and `duration_sec` (NOT duration_ms) match
+// the server contract. `year` is omitempty on the server so we keep it
+// nullable. CoverURL is non-null but may be "" — UI branches on isEmpty.
+class AlbumRef {
+  const AlbumRef({
+    required this.id,
+    required this.title,
+    required this.artistId,
+    this.sortTitle = '',
+    this.artistName = '',
+    this.year,
+    this.trackCount = 0,
+    this.durationSec = 0,
+    this.coverUrl = '',
+  });
+
+  final String id;
+  final String title;
+  final String sortTitle;
+  final String artistId;
+  final String artistName;
+  final int? year;
+  final int trackCount;
+  final int durationSec;
+  final String coverUrl;
+
+  factory AlbumRef.fromJson(Map<String, dynamic> j) => AlbumRef(
+        id: j['id'] as String,
+        title: j['title'] as String,
+        sortTitle: j['sort_title'] as String? ?? '',
+        artistId: j['artist_id'] as String,
+        artistName: j['artist_name'] as String? ?? '',
+        year: (j['year'] as num?)?.toInt(),
+        trackCount: (j['track_count'] as num?)?.toInt() ?? 0,
+        durationSec: (j['duration_sec'] as num?)?.toInt() ?? 0,
+        coverUrl: j['cover_url'] as String? ?? '',
+      );
+}