perf(player): commit seek gestures once after scrubbing

lib/widgets/playback_seek_slider.dart

@@ -0,0 +1,53 @@
+import 'package:flutter/material.dart';
+
+/// Previews scrubbing locally and seeks once when the gesture ends.
+class PlaybackSeekSlider extends StatefulWidget {
+  final Duration position;
+  final Duration duration;
+  final Future<void> Function(Duration) onSeek;
+
+  const PlaybackSeekSlider({
+    super.key,
+    required this.position,
+    required this.duration,
+    required this.onSeek,
+  });
+
+  @override
+  State<PlaybackSeekSlider> createState() => _PlaybackSeekSliderState();
+}
+
+class _PlaybackSeekSliderState extends State<PlaybackSeekSlider> {
+  double? _previewMs;
+  int _gestureGeneration = 0;
+
+  Future<void> _commit(double value) async {
+    final generation = _gestureGeneration;
+    try {
+      await widget.onSeek(Duration(milliseconds: value.round()));
+    } finally {
+      if (mounted && generation == _gestureGeneration) {
+        setState(() => _previewMs = null);
+      }
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final durationMs = widget.duration.inMilliseconds;
+    final enabled = durationMs > 0;
+    final maxMs = enabled ? durationMs.toDouble() : 1.0;
+    return Slider(
+      value: enabled
+          ? (_previewMs ?? widget.position.inMilliseconds.toDouble()).clamp(
+              0,
+              maxMs,
+            )
+          : 0,
+      max: maxMs,
+      onChangeStart: enabled ? (_) => _gestureGeneration++ : null,
+      onChanged: enabled ? (value) => setState(() => _previewMs = value) : null,
+      onChangeEnd: enabled ? _commit : null,
+    );
+  }
+}