feat(sidebar): multi-level playlist folders with tree and navigation views (#2017)

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.

src/renderer/features/settings/components/settings-option.module.css

@@ -0,0 +1,9 @@
+.row {
+    align-items: center;
+}
+
+.row-indented {
+    padding-left: var(--theme-spacing-lg);
+    margin-left: var(--theme-spacing-md);
+    border-left: 2px solid var(--theme-colors-surface);
+}