feat(jellyfin): songs and their lyrics, read as subtitles are

A music library's artists and albums open as shows and seasons do, and
can be tied to a studyset. A song plays as sound alone (`/Audio/{id}/
stream`, as stored) with its lyrics as Jellyfin keeps them (`/Audio/{id}/
Lyrics`: an LRC file beside it, or its tags), cut into the dictionary's
words as a video's subtitles are, with no translation. Timed lyrics
follow the song: the line sung large and bright among the others faded,
scrolled to as it comes, a line tapped played from its start; untimed
ones show whole. A song without lyrics shows greyed and doesn't open
(`HasLyrics`), as there is nothing in it to read. Its cover stands where
the picture would, glowing as a video does, and its words are tagged
with the artist, shown, and "artist · title", hidden.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/test/support/fake_jellyfin.dart

@@ -22,6 +22,9 @@ class FakeJellyfinGateway({
   final Map<String, List<JellyfinItem>> tree = const {},
   final Map<String, JellyfinPlayback> playbacks = const {},
   final Map<(String, int), String> vtt = const {},
+
+  /// Each song's lyrics, by its id.
+  final Map<String, List<({String text, Duration? start})>> lyricsOf = const {},
 }) implements JellyfinGateway {
   /// When set, every request after signing in fails with it.
   JellyfinProblem? fail;
@@ -172,6 +175,15 @@ class FakeJellyfinGateway({
     return vtt[(playback.itemId, track.index)] ?? 'WEBVTT\n';
   }
 
+  @override
+  Future<List<({String text, Duration? start})>> lyrics(
+    JellyfinAccount account,
+    String itemId,
+  ) async {
+    _check(account);
+    return lyricsOf[itemId] ?? const [];
+  }
+
   @override
   Uri imageOf(JellyfinAccount account, JellyfinItem item, {int height = 360}) =>
       fakeJellyfinServer.address.replace(path: '/Items/${item.id}/Images');