feat(ui): 歌曲列表改用封面图，替代统一的小图标

- 新增 SongArtwork 组件：内存字节 > 本地封面文件 > 图标兜底，圆角裁剪
- WebDAV 文件列表：前导图标换成封面（来自 songs.artwork_path）
- 播放列表页：批量查封面路径（getSongsByPaths），正在播放的歌优先用内存里的封面字节；
  切歌后重新拉一次，保证新补全的封面能显示
- 没有封面的歌仍回退为音符/播放图标，不影响列表结构

lib/pages/playlist_page.dart

@@ -2,6 +2,8 @@
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 import '../services/audio_service.dart';
+import '../database/song_database.dart';
+import '../widgets/song_artwork.dart';
 
 class PlaylistPage extends StatefulWidget {
   const PlaylistPage({super.key});
@@ -17,6 +19,9 @@ class _PlaylistPageState extends State<PlaylistPage> {
   int _scrollGeneration = 0;
   bool _hasScrolledToCurrent = false;
 
+  /// path -> 封面文件路径（来自 songs 表）
+  Map<String, String> _artworkByPath = {};
+
   // ⭐ 固定 item 高度（根据实际 ListTile 测量结果调整）
   // 实际约 72-76px，取 74px 作为稳定值
   static const double _itemExtent = 74.0;
@@ -28,13 +33,46 @@ class _PlaylistPageState extends State<PlaylistPage> {
   @override
   void initState() {
     super.initState();
+    _loadArtworkPaths();
+    AudioService().addListener(_onAudioChanged);
     WidgetsBinding.instance.addPostFrameCallback((_) {
       _scrollToCurrentSong();
     });
   }
 
+  String? _lastArtworkSongId;
+
+  /// 切歌后可能有新歌补全了封面，重新拉一次路径表
+  void _onAudioChanged() {
+    final song = AudioService().currentSong;
+    if (song == null || song.id == _lastArtworkSongId) return;
+    _lastArtworkSongId = song.id;
+    _loadArtworkPaths();
+  }
+
+  /// 批量取封面文件路径，供列表显示
+  Future<void> _loadArtworkPaths() async {
+    try {
+      final queue = AudioService().queue;
+      if (queue.isEmpty) return;
+      final rows =
+          await SongDatabase().getSongsByPaths(queue.map((s) => s.id).toList());
+      final map = <String, String>{};
+      rows.forEach((path, row) {
+        final artwork = row['artwork_path'] as String?;
+        if (artwork != null && artwork.isNotEmpty) map[path] = artwork;
+      });
+      if (mounted && map.isNotEmpty) {
+        setState(() => _artworkByPath = map);
+      }
+    } catch (_) {
+      // 封面只是装饰，失败不影响列表
+    }
+  }
+
   @override
   void dispose() {
+    AudioService().removeListener(_onAudioChanged);
     _scrollController.dispose();
     super.dispose();
   }
@@ -271,11 +309,15 @@ class _PlaylistPageState extends State<PlaylistPage> {
                     horizontal: 8,
                     vertical: 4,
                   ),
-                  leading: Icon(
-                    isCurrent ? Icons.play_arrow : Icons.music_note,
-                    color:
-                        isCurrent ? const Color(0xFFB8D4D0) : Colors.grey[600],
-                    size: 24,
+                  leading: SongArtwork(
+                    bytes: song.artwork,
+                    filePath: _artworkByPath[song.id],
+                    size: 48,
+                    fallbackIcon:
+                        isCurrent ? Icons.play_arrow : Icons.music_note,
+                    fallbackColor: isCurrent
+                        ? const Color(0xFFB8D4D0)
+                        : Colors.grey[600],
                   ),
                   title: Text(
                     song.title,