feat(lyrics): add countdown dots for instrumental breaks

lib/widgets/lyric_gap_indicator.dart

@@ -0,0 +1,60 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:spotiflac_android/l10n/l10n.dart';
+import 'package:spotiflac_android/providers/music_player_provider.dart';
+import 'package:spotiflac_android/utils/synced_lyrics_scroll.dart';
+
+/// A playback-driven countdown, rather than a repeating loading animation.
+class LyricGapIndicator extends ConsumerWidget {
+  const LyricGapIndicator({
+    super.key,
+    required this.start,
+    required this.end,
+    required this.color,
+  });
+
+  final Duration start;
+  final Duration end;
+  final Color color;
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final progress = syncedLyricSegmentProgress(
+      position: ref.watch(playbackPositionProvider),
+      start: start,
+      end: end,
+    );
+    final animate =
+        ref.watch(playbackPlayingProvider) &&
+        !ref.watch(playbackLoadingProvider) &&
+        !MediaQuery.disableAnimationsOf(context);
+    return Semantics(
+      label: context.l10n.nowPlayingInstrumental,
+      value: '${(progress * 100).round()}%',
+      child: Row(
+        mainAxisSize: MainAxisSize.min,
+        children: [
+          for (var index = 0; index < 3; index++) ...[
+            if (index > 0) const SizedBox(width: 8),
+            AnimatedContainer(
+              key: ValueKey('lyric-gap-dot-$index'),
+              duration: animate
+                  ? const Duration(milliseconds: 120)
+                  : Duration.zero,
+              width: 10,
+              height: 10,
+              decoration: BoxDecoration(
+                shape: BoxShape.circle,
+                color: Color.lerp(
+                  color.withValues(alpha: 0.25),
+                  color,
+                  (progress * 3 - index).clamp(0.0, 1.0),
+                ),
+              ),
+            ),
+          ],
+        ],
+      ),
+    );
+  }
+}