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/netease.ts

@@ -3,6 +3,7 @@ import type {
   MusicProvider,
   Song,
   Playlist,
+  PlaylistDetail,
   LyricLine,
   SearchResult,
   QrCodeResult,
@@ -319,6 +320,21 @@ export class NeteaseProvider implements MusicProvider {
     }));
   }
 
+  async getPlaylistDetail(playlistId: string): Promise<PlaylistDetail | null> {
+    const res = await this.api.get("/playlist/detail", {
+      params: { id: playlistId, ...this.cookieParams },
+    });
+    const p = res.data?.playlist;
+    if (!p) return null;
+    return {
+      id: String(p.id),
+      name: p.name ?? "",
+      description: p.description ?? "",
+      coverUrl: p.coverImgUrl ?? "",
+      songCount: p.trackCount ?? 0,
+    };
+  }
+
   async getUserPlaylists(): Promise<Playlist[]> {
     // First get user ID from login status
     const statusRes = await this.api.get("/login/status", {