fix(playback): 歌单/文件列表播放时标题不再显示百分号编码

播放路径是编码的，无元数据时直接截取文件名会显示
"%E6%9C%AA%E5%88%86..." 这种串。

- playPlaylist：统一解码后再去扩展名，DB 标题为空也回退到解码名
- PlaybackController：单曲与队列构建同样走 _displayName()
- 有标签时仍优先用标签标题

lib/services/audio_service.dart

@@ -300,23 +300,29 @@ class AudioService extends ChangeNotifier {
     final songs = <Song>[];
     for (final id in songIds) {
       final dbSong = await _db.getSongByPath(id);
-      if (dbSong != null) {
-        final path = dbSong['remote_path'] as String;
-        songs.add(Song(
-          id: path,
-          artworkPath: dbSong['artwork_path'] as String?,
-          title: dbSong['title'] as String? ?? '',
-          artist: dbSong['artist'] as String? ?? '未知艺术家',
-          url: WebDAVService.instance.getFileUrl(path),
-        ));
-      } else {
-        songs.add(Song(
-          id: id,
-          title: id.split('/').last.replaceAll(RegExp(r'\.[^.]*$'), ''),
-          artist: '未知艺术家',
-          url: WebDAVService.instance.getFileUrl(id),
-        ));
+      final path = (dbSong?['remote_path'] as String?) ?? id;
+
+      // 没有标签时回退文件名，必须解码：id 是百分号编码的播放路径，
+      // 直接展示会变成 "%E6%9C%AA..." 这种不可读串。
+      final rawName = path.split('/').last;
+      String displayName;
+      try {
+        displayName = Uri.decodeComponent(rawName);
+      } catch (_) {
+        displayName = rawName;
       }
+      displayName = displayName.replaceAll(RegExp(r'\.[^.]*$'), '');
+
+      final dbTitle = (dbSong?['title'] as String?)?.trim() ?? '';
+      final dbArtist = (dbSong?['artist'] as String?)?.trim() ?? '';
+
+      songs.add(Song(
+        id: path,
+        artworkPath: dbSong?['artwork_path'] as String?,
+        title: dbTitle.isNotEmpty ? dbTitle : displayName,
+        artist: dbArtist.isNotEmpty ? dbArtist : '未知艺术家',
+        url: WebDAVService.instance.getFileUrl(path),
+      ));
     }
 
     if (songs.isNotEmpty) {