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/qq.ts

@@ -53,6 +53,7 @@ export function mapQqSongs(raw: any[] | null | undefined): Song[] {
         ? `https://y.gtimg.cn/music/photo_new/T002R300x300M000${albumMid}.jpg`
         : "",
       platform: "qq" as const,
+      vip: s.pay?.payplay === 1 || s.pay?.paytrackprice === 1 || false,
     };
   }).filter((s) => s.id);
 }
@@ -520,6 +521,7 @@ export class QQMusicProvider implements MusicProvider {
           ? `https://y.gtimg.cn/music/photo_new/T002R300x300M000${s.album.mid}.jpg`
           : "",
         platform: "qq",
+        vip: s.pay?.payplay === 1 || s.pay?.paytrackprice === 1 || false,
       }));
     } catch {
       return [];