MetroList: Preserve Downloaded Songs During Sync

Problem

Downloaded songs were being removed from playlists during sync, even though they were available offline. The sync logic didn't check download status before cleaning up playlist entries.

Solution

Added isDownloaded property to the Song class. Modified sync logic to preserve songs with either isDownloaded set or dateDownload set. Updated sync preservation checks across all sync entry points.

Result

Downloaded songs remain in playlists after sync. Users can rely on their offline content being preserved.

Commit: MetrolistGroup/Metrolist@cd72309
PR: #3436