feat(S04/T01): Added DiscNumber field to MBTrack (populated from Medium…

- backend/explore/types.go
- backend/explore/musicbrainz.go
- frontend/wailsjs/go/explore/Service.d.ts
- frontend/index.ts

frontend/wailsjs/go/explore/Service.d.ts

@@ -41,6 +41,7 @@ export interface MBRelease {
 
 export interface MBTrack {
   position: number;
+  discNumber: number;
   title: string;
   length: number;
   mbid: string;