Playlist multi selection

[timusus]

Implements multi-select feature for playlists as requested in issue #84. Users can now select multiple songs within a playlist and perform batch operations like adding to queue or moving to another playlist.

Changes:

• Added contextual toolbar to playlist detail layout
• Integrated ContextualToolbarHelper for managing multi-select state
• Updated click/long-click handlers to support multi-select mode
• Added batch operation methods to PlaylistDetailPresenter:
  • addToQueue(List<PlaylistSong>) for adding multiple songs to queue
  • editTags(List<PlaylistSong>) for editing tags on multiple songs
• Contextual toolbar shows "Add to Queue", "Add to Playlist", and "Edit Tags" options when songs are selected

Long-press on a song activates multi-select mode, then tap additional songs to toggle their selection. The contextual toolbar displays the count of selected items and provides batch action options.

Fixes #84