feat: expose vip flag on Song for trial-only detection

Add optional vip?: boolean to the Song interface so downstream clients
(e.g. PowerfulTS) can mark copyright-restricted songs that non-VIP users
can only play as a trial fragment, before playback starts.

- provider.ts: add optional vip?: boolean (backward compatible)
- netease.ts: extract mapNeteaseSongs() pure fn; map fee to vip
  (1=VIP, 4=album-only). fee=8 (free low-quality) is excluded because
  it plays in full, just at lower quality.
- qq.ts: mapQqSongs() maps pay.payplay/paytrackprice to vip (one fix
  covers all callers); getDailyRecommendSongs inline mapping too.
- tests: vip mapping assertions for netease fee (1/4=vip, 0/8=free) and
  qq pay fields.

src/music/provider.ts

@@ -6,6 +6,9 @@ export interface Song {
   duration: number; // seconds
   coverUrl: string;
   platform: "netease" | "qq" | "bilibili" | "youtube";
+  /** VIP / copyright-restricted: non-VIP users can only play a trial fragment
+   *  (NetEase fee=1 VIP / fee=4 album-only, or QQ pay.payplay/paytrackprice=1). */
+  vip?: boolean;
 }
 
 export interface SongWithUrl extends Song {