refactor: consolidate client requests and move currently_playing_tracks_id to PlayerState

[aome510]

Summary

This PR simplifies the client request model by consolidating multiple specialized track-fetching requests into the single existing GetContext request, and moves currently_playing_tracks_id from UIState to PlayerState.

Consolidate ClientRequest variants

Removed GetUserSavedTracks, GetUserTopTracks, GetUserRecentlyPlayedTracks, and GetRadioTracks request variants. All of these are now dispatched as GetContext(ContextId::Tracks(...)). The handler resolves the appropriate API call by pattern-matching on the tracks URI string (USER_TOP_TRACKS_URI, USER_RECENTLY_PLAYED_TRACKS_URI, USER_LIKED_TRACKS_URI, or a radio: prefixed URI).

Move currently_playing_tracks_id to PlayerState

The field tracking the active Tracks context (for contexts not represented in Spotify's playback API, e.g. liked/top tracks) was moved from UIState to PlayerState. This makes it accessible to PlayerState::playing_context_id(), which now falls back to currently_playing_tracks_id when there is no active Spotify context. All event handler functions that modify this field were updated to take &SharedState instead of using UIStateGuard.