style(ui): 列表行距收紧 + 收藏项显示封面

- 收藏列表：vertical padding 6 -> 2，ListTile 用 compact，
  标题 17->16、副标题 14->13，行高明显变紧凑
- 收藏项 leading 从音符图标换成 SongArtwork（复用 songs.artwork_path）
- WebDAV 文件列表：contentPadding 去掉 vertical 2，加 compact

真机截图确认：行距更接近设计稿，有元数据的歌显示封面。

lib/pages/webdav_file_list_page.dart

@@ -351,7 +351,8 @@ class _WebDAVFileListPageState extends BaseState<WebDAVFileListPage> {
 
     if (item.isDirectory) {
       return ListTile(
-        contentPadding: const EdgeInsets.symmetric(horizontal: 4, vertical: 2),
+        contentPadding: const EdgeInsets.symmetric(horizontal: 4),
+        visualDensity: VisualDensity.compact,
         leading: const Icon(
           Icons.folder_outlined,
           color: Color(0xFFB8D4D0),
@@ -397,7 +398,8 @@ class _WebDAVFileListPageState extends BaseState<WebDAVFileListPage> {
       ].join(' · ');
 
       return ListTile(
-        contentPadding: const EdgeInsets.symmetric(horizontal: 4, vertical: 2),
+        contentPadding: const EdgeInsets.symmetric(horizontal: 4),
+        visualDensity: VisualDensity.compact,
         leading: SongArtwork(
           filePath: meta?['artwork_path'] as String?,
           size: 44,