fix(player): 记住拖动后的实际播放位置

lib/core/data/services/playback_position_cache.dart

@@ -2,6 +2,15 @@
 
 const Duration _defaultMaxAge = Duration(hours: 12);
 
+/// 将播放位置限制在当前媒体时长内。
+Duration clampPlaybackPosition(Duration position, Duration duration) {
+  if (position <= Duration.zero || duration <= Duration.zero) {
+    return Duration.zero;
+  }
+  if (position >= duration) return duration;
+  return position;
+}
+
 /// 进程内最近播放位置。
 class CachedPlaybackPosition {
   const CachedPlaybackPosition({
@@ -37,13 +46,14 @@ class PlaybackPositionCache {
     required Duration position,
     required Duration duration,
   }) {
-    if (episodeId.isEmpty || dramaId.isEmpty || position <= Duration.zero) {
+    final safePosition = clampPlaybackPosition(position, duration);
+    if (episodeId.isEmpty || dramaId.isEmpty || safePosition <= Duration.zero) {
       return;
     }
     _positions[episodeId] = CachedPlaybackPosition(
       episodeId: episodeId,
       dramaId: dramaId,
-      position: position,
+      position: safePosition,
       duration: duration,
       updatedAt: _now(),
     );

lib/core/data/services/video_player_port.dart

@@ -61,6 +61,9 @@ class VideoPlayerPort implements PlayerPort {
     if (!_disposed && controller.value.isInitialized) await controller.pause();
   }
 
+  /// 跳转到指定播放位置。
+  Future<void> seekTo(Duration position) => controller.seekTo(position);
+
   @override
   Future<void> dispose() async {
     if (_disposed) return;