feat(lyrics): add countdown dots for instrumental breaks

lib/utils/lyrics_timeline.dart

@@ -0,0 +1,45 @@
+import 'package:spotiflac_android/utils/lyrics_parser.dart';
+
+/// Inserts empty, bounded rows for instrumental countdowns in the player.
+/// Unmarked gaps in plain LRC cannot be distinguished from held vocals: only
+/// use explicit empty timestamps or known line/word ends, never an estimate.
+List<LyricLine> lyricsTimelineWithGaps(List<LyricLine> lines) {
+  const minimumGap = Duration(seconds: 3);
+  final timeline = <LyricLine>[];
+  Duration? gapStart;
+  Duration? latestVocalEnd;
+
+  for (final line in lines) {
+    if (line.text.trim().isEmpty) {
+      gapStart ??= line.time;
+      continue;
+    }
+
+    var start = timeline.isEmpty ? Duration.zero : gapStart;
+    // A second vocal line may overlap a previous, longer one.
+    if (start != null && latestVocalEnd != null && start < latestVocalEnd) {
+      start = latestVocalEnd;
+    }
+    if (start != null && line.time - start >= minimumGap) {
+      timeline.add(LyricLine(time: start, end: line.time, text: ''));
+    }
+    timeline.add(line);
+
+    final lastWord = line.words.lastOrNull;
+    var end = line.end;
+    if (lastWord != null) {
+      if (lastWord.end != null && (end == null || lastWord.end! > end)) {
+        end = lastWord.end;
+      }
+      if (end != null && end < lastWord.time) end = null;
+    }
+    if (end != null && end < line.time) end = null;
+    gapStart = end;
+    if (end != null && (latestVocalEnd == null || end > latestVocalEnd)) {
+      latestVocalEnd = end;
+    }
+  }
+  // No following vocal means no countdown: trailing blank timestamps are
+  // intentionally omitted, and credits stay after the last sung line.
+  return List.unmodifiable(timeline);
+}