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/provider.ts

@@ -20,6 +20,14 @@ export interface Playlist {
   platform: "netease" | "qq" | "bilibili" | "youtube";
 }
 
+export interface PlaylistDetail {
+  id: string;
+  name: string;
+  description: string;
+  coverUrl: string;
+  songCount: number;
+}
+
 export interface Album {
   id: string;
   name: string;
@@ -77,4 +85,5 @@ export interface MusicProvider {
   getPersonalFm?(): Promise<Song[]>;
   getDailyRecommendSongs?(): Promise<Song[]>;
   getUserPlaylists?(): Promise<Playlist[]>;
+  getPlaylistDetail?(playlistId: string): Promise<PlaylistDetail | null>;
 }