feat: offline skip for uncached songs, cache backup & restore with full offline playback, and crash fixes

[drkvenom786]

Overview

This PR introduces offline playback enhancements, cache backup & restoration, crash fixes, and general improvements.

Changes Included

1. Offline Skip for Uncached Songs:
   • Added a setting under Player & Audio settings to skip tracks that are not cached when offline, preventing stalled buffering or unexpected pauses.
2. Cache Backup & Restore:
   • Implemented backup and restore for cached songs and playback metadata.
   • Properly aligns ExoPlayer cache UID and version registry (ExoPlayerVersions) to ensure restored .exo chunk files are preserved and never purged on startup.
   • Reconstructs cache index mapping if restoring older backups.
3. Full Offline Audio Playback:
   • Relaxed data source cache checks in MusicService.kt and DownloadUtil.kt so tracks with cached audio bytes play immediately offline from local storage without network requests.
   • Added reactive cache tracking in CachePlaylistScreen and StorageSettings so restored songs appear immediately in the Cached playlist.
4. Crash Fixes:
   • Fixed NoSuchElementException: Collection is empty crashes in OnlinePlaylistAdder.kt and YouTubeAlbumRadio.kt.