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/home_data.dart

@@ -0,0 +1,46 @@
+import 'album.dart';
+import 'artist.dart';
+import 'track.dart';
+
+// Mirrors internal/api/types.go HomePayload and web/src/lib/api/types.ts.
+// Section keys MUST stay in sync with the server's JSON tags:
+//   recently_added_albums / rediscover_albums / rediscover_artists /
+//   most_played_tracks / last_played_artists.
+//
+// Server contract guarantees all five slices are non-nil at JSON encode
+// time (empty sections render as []), but we still tolerate missing keys
+// so partial fixtures and offline-degraded responses can be parsed.
+class HomeData {
+  const HomeData({
+    required this.recentlyAddedAlbums,
+    required this.rediscoverAlbums,
+    required this.rediscoverArtists,
+    required this.mostPlayedTracks,
+    required this.lastPlayedArtists,
+  });
+
+  final List<AlbumRef> recentlyAddedAlbums;
+  final List<AlbumRef> rediscoverAlbums;
+  final List<ArtistRef> rediscoverArtists;
+  final List<TrackRef> mostPlayedTracks;
+  final List<ArtistRef> lastPlayedArtists;
+
+  factory HomeData.fromJson(Map<String, dynamic> j) => HomeData(
+        recentlyAddedAlbums: _list(j, 'recently_added_albums', AlbumRef.fromJson),
+        rediscoverAlbums:    _list(j, 'rediscover_albums',     AlbumRef.fromJson),
+        rediscoverArtists:   _list(j, 'rediscover_artists',    ArtistRef.fromJson),
+        mostPlayedTracks:    _list(j, 'most_played_tracks',    TrackRef.fromJson),
+        lastPlayedArtists:   _list(j, 'last_played_artists',   ArtistRef.fromJson),
+      );
+
+  static List<T> _list<T>(
+    Map<String, dynamic> j,
+    String key,
+    T Function(Map<String, dynamic>) parse,
+  ) {
+    final raw = j[key] as List? ?? const [];
+    return raw
+        .map((e) => parse((e as Map).cast<String, dynamic>()))
+        .toList(growable: false);
+  }
+}