fix(api): add getPlaylistDetail provider method for QQ playlist detail

The /playlist/:id/detail route was hardcoded to call the NetEase API
path /playlist/detail on whatever provider was selected. For QQ this
hit a non-existent path, so clicking into any QQ playlist showed
"歌单不存在或加载失败".

Replace the platform-specific hack in the route handler with a proper
getPlaylistDetail method on MusicProvider. Implement it for NetEase
(porting the existing /playlist/detail logic) and QQ (calling
/getSongListDetail and reading from response.cdlist[0]).

Pre-existing bug exposed by the QQ source tab.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/music/qq.ts

@@ -3,6 +3,7 @@ import type {
   MusicProvider,
   Song,
   Playlist,
+  PlaylistDetail,
   LyricLine,
   SearchResult,
   QrCodeResult,
@@ -178,6 +179,21 @@ export class QQMusicProvider implements MusicProvider {
     }));
   }
 
+  async getPlaylistDetail(playlistId: string): Promise<PlaylistDetail | null> {
+    const res = await this.api.get("/getSongListDetail", {
+      params: { disstid: playlistId, ...this.cookieParams },
+    });
+    const cd = res.data?.response?.cdlist?.[0];
+    if (!cd) return null;
+    return {
+      id: String(cd.disstid ?? cd.dissid ?? ""),
+      name: cd.dissname ?? "",
+      description: cd.desc ?? "",
+      coverUrl: cd.logo ?? "",
+      songCount: cd.songnum ?? cd.total_song_num ?? 0,
+    };
+  }
+
   async getRecommendPlaylists(): Promise<Playlist[]> {
     const res = await this.api.get("/getSongLists", {
       params: { categoryId: 10000000, pageSize: 10, ...this.cookieParams },