feat(music): add music data layer for plex and jellyfin

Backend-neutral fetchArtistAlbums/fetchAlbumTracks/fetchInstantMix/
fetchLyrics, lyrics model + LRC parser, audio quality presets, music
transcode (plex /music universal, jellyfin audio PlaybackInfo profile),
station/InstantMix radio, music search, playlist type derivation, and
audio download resolution.

lib/utils/lrc_parser.dart

@@ -0,0 +1,63 @@
+import '../media/lyrics.dart';
+
+/// Matches `[mm:ss]`, `[mm:ss.x]`, `[mm:ss.xx]`, `[mm:ss.xxx]` timestamps
+/// (multiple per line allowed, per the LRC spec).
+final RegExp _timestamp = RegExp(r'\[(\d{1,3}):(\d{1,2})(?:\.(\d{1,3}))?\]');
+
+/// Metadata/ID tags like `[ar:Artist]`, `[offset:+120]`.
+final RegExp _idTag = RegExp(r'^\[([a-zA-Z#]+):(.*)\]$');
+
+/// Parse LRC content into [Lyrics]. Lines with `[mm:ss.xx]` timestamps become
+/// synced lines (a line with multiple timestamps is emitted once per
+/// timestamp); the `[offset:±ms]` tag is applied. Input with no timestamps at
+/// all falls back to unsynced plain text. Returns `null` for blank input.
+Lyrics? parseLrc(String raw) {
+  final lines = <LyricLine>[];
+  final plain = <String>[];
+  var offsetMs = 0;
+
+  for (final rawLine in raw.split(RegExp(r'\r?\n'))) {
+    final line = rawLine.trim();
+    if (line.isEmpty) continue;
+
+    final idTag = _idTag.firstMatch(line);
+    if (idTag != null && !_timestamp.hasMatch(line)) {
+      if (idTag.group(1)!.toLowerCase() == 'offset') {
+        offsetMs = int.tryParse(idTag.group(2)!.trim()) ?? 0;
+      }
+      continue;
+    }
+
+    final stamps = _timestamp.allMatches(line).toList();
+    if (stamps.isEmpty) {
+      plain.add(line);
+      continue;
+    }
+
+    final text = line.substring(stamps.last.end).trim();
+    for (final m in stamps) {
+      final minutes = int.parse(m.group(1)!);
+      final seconds = int.parse(m.group(2)!);
+      // Fractional part scales by digit count: ".5" = 500ms, ".50" = 500ms.
+      final frac = m.group(3);
+      final fracMs = frac == null ? 0 : (int.parse(frac) * 1000 ~/ _pow10(frac.length)).clamp(0, 999);
+      final startMs = (minutes * 60 + seconds) * 1000 + fracMs - offsetMs;
+      lines.add(LyricLine(text: text, startMs: startMs < 0 ? 0 : startMs));
+    }
+  }
+
+  if (lines.isNotEmpty) {
+    lines.sort((a, b) => a.startMs!.compareTo(b.startMs!));
+    return Lyrics(synced: true, lines: lines);
+  }
+  if (plain.isNotEmpty) {
+    return Lyrics(synced: false, lines: [for (final t in plain) LyricLine(text: t)]);
+  }
+  return null;
+}
+
+int _pow10(int n) => switch (n) {
+  1 => 10,
+  2 => 100,
+  _ => 1000,
+};