feat(lyrics): animate live previews while scrubbing playback

lib/widgets/lyric_gap_indicator.dart

@@ -11,16 +11,18 @@ class LyricGapIndicator extends ConsumerWidget {
     required this.start,
     required this.end,
     required this.color,
+    this.position,
   });
 
   final Duration start;
   final Duration end;
   final Color color;
+  final Duration? position;
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final progress = syncedLyricSegmentProgress(
-      position: ref.watch(playbackPositionProvider),
+      position: position ?? ref.watch(playbackPositionProvider),
       start: start,
       end: end,
     );

lib/widgets/playback_seek_slider.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:spotiflac_android/theme/mornye_theme.dart';
+import 'package:spotiflac_android/utils/playback_seek_preview.dart';
 import 'package:spotiflac_android/widgets/mornye_player_slider.dart';
 
 /// Previews scrubbing locally and seeks once when the gesture ends.
@@ -7,12 +8,14 @@ class PlaybackSeekSlider extends StatefulWidget {
   final Duration position;
   final Duration duration;
   final Future<void> Function(Duration) onSeek;
+  final PlaybackSeekPreview? preview;
 
   const PlaybackSeekSlider({
     super.key,
     required this.position,
     required this.duration,
     required this.onSeek,
+    this.preview,
   });
 
   @override
@@ -22,18 +25,53 @@ class PlaybackSeekSlider extends StatefulWidget {
 class _PlaybackSeekSliderState extends State<PlaybackSeekSlider> {
   double? _previewMs;
   int _gestureGeneration = 0;
+  Object? _previewSession;
+
+  void _start(double value) {
+    _gestureGeneration++;
+    _previewSession = widget.preview?.begin(
+      Duration(milliseconds: value.round()),
+    );
+  }
+
+  void _preview(double value) {
+    setState(() => _previewMs = value);
+    final session = _previewSession;
+    if (session != null) {
+      widget.preview?.update(session, Duration(milliseconds: value.round()));
+    }
+  }
+
+  Future<void> _adjust(double value) {
+    _start(value);
+    _preview(value);
+    return _commit(value);
+  }
 
   Future<void> _commit(double value) async {
     final generation = _gestureGeneration;
+    final preview = widget.preview;
+    final session = _previewSession;
     try {
       await widget.onSeek(Duration(milliseconds: value.round()));
     } finally {
+      preview?.end(session);
       if (mounted && generation == _gestureGeneration) {
         setState(() => _previewMs = null);
       }
     }
   }
 
+  @override
+  void dispose() {
+    final preview = widget.preview;
+    final session = _previewSession;
+    // Rotation or track replacement can remove this slider during layout.
+    // Notify surviving lyrics/time labels after that frame has finished.
+    WidgetsBinding.instance.addPostFrameCallback((_) => preview?.end(session));
+    super.dispose();
+  }
+
   @override
   Widget build(BuildContext context) {
     final durationMs = widget.duration.inMilliseconds;
@@ -58,10 +96,10 @@ class _PlaybackSeekSliderState extends State<PlaybackSeekSlider> {
               ? percentage((currentMs - 5000).clamp(0, maxMs).toDouble())
               : null,
           onIncrease: enabled
-              ? () => _commit((currentMs + 5000).clamp(0, maxMs).toDouble())
+              ? () => _adjust((currentMs + 5000).clamp(0, maxMs).toDouble())
               : null,
           onDecrease: enabled
-              ? () => _commit((currentMs - 5000).clamp(0, maxMs).toDouble())
+              ? () => _adjust((currentMs - 5000).clamp(0, maxMs).toDouble())
               : null,
           child: IgnorePointer(
             ignoring: !enabled,
@@ -72,8 +110,8 @@ class _PlaybackSeekSliderState extends State<PlaybackSeekSlider> {
                   SliderTheme.of(context).inactiveTrackColor ?? Colors.white12,
               value: enabled ? currentMs : 0,
               max: maxMs,
-              onChangeStart: (_) => _gestureGeneration++,
-              onChanged: (value) => setState(() => _previewMs = value),
+              onChangeStart: _start,
+              onChanged: _preview,
               onChangeEnd: _commit,
             ),
           ),
@@ -88,8 +126,8 @@ class _PlaybackSeekSliderState extends State<PlaybackSeekSlider> {
             )
           : 0,
       max: maxMs,
-      onChangeStart: enabled ? (_) => _gestureGeneration++ : null,
-      onChanged: enabled ? (value) => setState(() => _previewMs = value) : null,
+      onChangeStart: enabled ? _start : null,
+      onChanged: enabled ? _preview : null,
       onChangeEnd: enabled ? _commit : null,
     );
   }