feat(flutter): playlists list + detail screens

- models/playlist.dart (Playlist, PlaylistTrack, PlaylistDetail)
- api/endpoints/playlists.dart (list with kind=user|system|all, get detail)
- playlists/playlists_provider.dart (Riverpod family providers)
- playlists/playlists_list_screen.dart (with system-variant pill)
- playlists/playlist_detail_screen.dart (header + Play button + rows)
- /playlists + /playlists/:id routes wired
- Home AppBar: queue_music icon next to Search

Tracks with track_id=null render greyed-out + struck-through (matches
web's PlaylistTrackRow behavior). Tap a row plays from that position;
top-level Play button plays the full playable subset from track 0.

flutter_client/lib/models/playlist.dart

@@ -0,0 +1,117 @@
+// Mirrors internal/api/playlists.go Playlist + PlaylistDetail wire shapes.
+//
+// Server distinguishes user playlists (created by the caller) from
+// system playlists (server-generated mixes like for_you / discover).
+// system_variant is null for user playlists; "for_you" / "discover"
+// otherwise. Read-only operations work the same; PATCH/POST/DELETE
+// are gated on user-owned playlists.
+
+class Playlist {
+  const Playlist({
+    required this.id,
+    required this.userId,
+    required this.name,
+    required this.description,
+    required this.isPublic,
+    required this.systemVariant,
+    required this.trackCount,
+    required this.coverUrl,
+    required this.ownerUsername,
+    required this.createdAt,
+    required this.updatedAt,
+  });
+
+  final String id;
+  final String userId;
+  final String name;
+  final String description;
+  final bool isPublic;
+  /// "for_you" / "discover" / null
+  final String? systemVariant;
+  final int trackCount;
+  /// Server-emitted URL to the cover; empty when no tracks yet.
+  final String coverUrl;
+  /// Display name of the owner — for showing other users' public playlists.
+  final String ownerUsername;
+  final String createdAt;
+  final String updatedAt;
+
+  bool get isSystem => systemVariant != null;
+
+  factory Playlist.fromJson(Map<String, dynamic> j) => Playlist(
+        id: j['id'] as String,
+        userId: j['user_id'] as String? ?? '',
+        name: j['name'] as String? ?? '',
+        description: j['description'] as String? ?? '',
+        isPublic: j['is_public'] as bool? ?? false,
+        systemVariant: j['system_variant'] as String?,
+        trackCount: (j['track_count'] as num?)?.toInt() ?? 0,
+        coverUrl: j['cover_url'] as String? ?? '',
+        ownerUsername: j['owner_username'] as String? ?? '',
+        createdAt: j['created_at'] as String? ?? '',
+        updatedAt: j['updated_at'] as String? ?? '',
+      );
+}
+
+/// Playlist row as returned in PlaylistDetail.tracks. Each row carries
+/// the track's display fields directly so the client doesn't need a
+/// separate /api/tracks/{id} fetch per row.
+///
+/// track_id is nullable: when the upstream track has been removed from
+/// the library, the row remains in the playlist with its title/artist
+/// preserved but track_id=null and stream_url=null. UI should render
+/// these greyed-out and unplayable.
+class PlaylistTrack {
+  const PlaylistTrack({
+    required this.position,
+    required this.trackId,
+    required this.title,
+    required this.albumId,
+    required this.albumTitle,
+    required this.artistId,
+    required this.artistName,
+    required this.durationSec,
+    required this.streamUrl,
+  });
+
+  final int position;
+  final String? trackId;
+  final String title;
+  final String? albumId;
+  final String albumTitle;
+  final String? artistId;
+  final String artistName;
+  final int durationSec;
+  final String? streamUrl;
+
+  bool get isAvailable => trackId != null;
+
+  factory PlaylistTrack.fromJson(Map<String, dynamic> j) => PlaylistTrack(
+        position: (j['position'] as num?)?.toInt() ?? 0,
+        trackId: j['track_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? ?? '',
+        durationSec: (j['duration_sec'] as num?)?.toInt() ?? 0,
+        streamUrl: j['stream_url'] as String?,
+      );
+}
+
+class PlaylistDetail {
+  const PlaylistDetail({required this.playlist, required this.tracks});
+
+  final Playlist playlist;
+  final List<PlaylistTrack> tracks;
+
+  factory PlaylistDetail.fromJson(Map<String, dynamic> j) {
+    final tracksRaw = (j['tracks'] as List?) ?? const [];
+    return PlaylistDetail(
+      playlist: Playlist.fromJson(j),
+      tracks: tracksRaw
+          .map((e) => PlaylistTrack.fromJson((e as Map).cast<String, dynamic>()))
+          .toList(growable: false),
+    );
+  }
+}