Group playlists into folders using a configurable separator (default '/').
Three view modes:
- Single: first-level grouping only (original behavior)
- Tree: full recursive nesting with connecting lines (configurable indent and line color)
- Navigation: drill-down view with stacked breadcrumb chain
Folders are sorted before playlists at every level. New settings render as
indented sub-options under the master 'Enable folders' toggle.
jeffvli