MetroList: Cached Songs Globally Available

Problem

Cached songs were only available within specific contexts. The app didn't provide system-wide access to cached content for offline playback.

Solution

Made cached songs globally available through the app. Implemented proper cache completion checking on startup. Added UI restoration for cached playlists.

Result

Cached songs are accessible anywhere in the app. Offline playback works consistently across all features.

Commit: MetrolistGroup/Metrolist@1dfd746
PR: #3141