fix(playback): 播放页/迷你播放器立即显示已有封面，不再等元数据流程

原因有两层：
1) Song 模型没有封面路径，播放时只带 url/title/artist，
   播放页只能等元数据服务返回 artwork 字节后才显示；
2) 元数据回来后用新 Song 覆盖当前歌曲时丢了 artworkPath，
   即使带上了也会被冲掉。

- Song 增加 artworkPath 字段
- PlaybackController / 首页收藏 / playPlaylist 构造 Song 时带上
- 播放页：artwork 字节优先，其次直接读 artwork_path 文件
- 迷你播放器同样加 FileImage 兜底
- 元数据更新重建 Song 时保留 artworkPath

真机验证：点击播放后封面立即出现，不再先显示音符占位。

lib/controllers/playback_controller.dart

@@ -98,6 +98,7 @@ class PlaybackController {
     final dbSong = await _db.getSongByPath(songPath);
     final song = Song(
       id: songPath,
+      artworkPath: dbSong?['artwork_path'] as String?,
       title: dbSong?['title'] as String? ??
           songPath.split('/').last.replaceAll(RegExp(r'\.[^.]*$'), ''),
       artist: dbSong?['artist'] as String? ?? '未知艺术家',
@@ -117,6 +118,7 @@ class PlaybackController {
       if (dbSong != null) {
         queue.add(Song(
           id: item.path,
+          artworkPath: dbSong['artwork_path'] as String?,
           title: dbSong['title'] as String? ??
               item.name.replaceAll(RegExp(r'\.[^.]*$'), ''),
           artist: dbSong['artist'] as String? ?? '未知艺术家',

lib/pages/home_page.dart

@@ -378,6 +378,7 @@ class _HomePageState extends BaseState<HomePage> {
         id: song.path,
         title: song.displayTitle,
         artist: song.displaySubtitle,
+        artworkPath: song.artworkPath,
         url: url,
       ));
     } catch (e) {
@@ -619,6 +620,7 @@ class _HomePageState extends BaseState<HomePage> {
                 id: item.path,
                 title: item.displayTitle,
                 artist: item.displaySubtitle,
+                artworkPath: item.artworkPath,
                 url: WebDAVService.instance.getFileUrl(item.path),
               ))
           .toList();

lib/pages/player_page.dart

@@ -1,3 +1,4 @@
+import 'dart:io';
 // lib/pages/player_page.dart
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
@@ -322,6 +323,11 @@ class _PlayerPageState extends BaseState<PlayerPage> {
         ? (screen.height * 0.38).clamp(160.0, 380.0)
         : null;
 
+    final artworkPath = song.artworkPath;
+    final hasArtworkFile = artworkPath != null &&
+        artworkPath.isNotEmpty &&
+        File(artworkPath).existsSync();
+
     final cover = ClipRRect(
       borderRadius: BorderRadius.circular(compact ? 18 : 24),
       child: song.artwork != null
@@ -330,6 +336,12 @@ class _PlayerPageState extends BaseState<PlayerPage> {
               fit: BoxFit.cover,
               gaplessPlayback: true,
             )
+          : hasArtworkFile
+              ? Image.file(
+                  File(artworkPath),
+                  fit: BoxFit.cover,
+                  gaplessPlayback: true,
+                )
               : Container(
                   color: const Color(0xFF2A3332),
                   child: Icon(

lib/services/audio_service.dart

@@ -32,12 +32,17 @@ class Song {
   final String? url;
   final Uint8List? artwork;
 
+  /// 本地封面文件路径（songs.artwork_path）。
+  /// 命中过元数据的歌直接用它显示封面，不必等元数据流程回来。
+  final String? artworkPath;
+
   Song({
     required this.id,
     required this.title,
     required this.artist,
     this.url,
     this.artwork,
+    this.artworkPath,
   });
 
   Map<String, dynamic> toJson() => {
@@ -299,6 +304,7 @@ class AudioService extends ChangeNotifier {
         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),
@@ -855,6 +861,8 @@ class AudioService extends ChangeNotifier {
           artist: metadata.artist.isNotEmpty ? metadata.artist : song.artist,
           url: song.url,
           artwork: metadata.artwork,
+          // 保留封面文件路径：元数据流程回来前，播放页靠它立即显示封面
+          artworkPath: song.artworkPath,
         );
         _queue[_currentIndex] = updatedSong;
         _currentSong = updatedSong;

lib/widgets/global_mini_player.dart

@@ -1,3 +1,4 @@
+import 'dart:io';
 // lib/widgets/global_mini_player.dart
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
@@ -54,7 +55,6 @@ class GlobalMiniPlayer extends StatelessWidget {
                     borderRadius: BorderRadius.circular(12),
                     child: Row(
                       children: [
-                        const SizedBox(width: 12),
                         Container(
                           width: 44,
                           height: 44,
@@ -66,9 +66,22 @@ class GlobalMiniPlayer extends StatelessWidget {
                                     image: MemoryImage(song.artwork!),
                                     fit: BoxFit.cover,
                                   )
+                                : (song.artworkPath != null &&
+                                        song.artworkPath!.isNotEmpty &&
+                                        File(song.artworkPath!)
+                                            .existsSync())
+                                    ? DecorationImage(
+                                        image: FileImage(
+                                            File(song.artworkPath!)),
+                                        fit: BoxFit.cover,
+                                      )
                                     : null,
                           ),
-                          child: song.artwork == null
+                          child: (song.artwork == null &&
+                                  (song.artworkPath == null ||
+                                      song.artworkPath!.isEmpty ||
+                                      !File(song.artworkPath!)
+                                          .existsSync()))
                               ? const Icon(
                                   Icons.music_note,
                                   color: Colors.white38,