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/shared/components/icon/icon.tsx

@@ -34,6 +34,8 @@ import {
     LuChevronLast,
     LuChevronLeft,
     LuChevronRight,
+    LuChevronsDownUp,
+    LuChevronsUpDown,
     LuChevronUp,
     LuCircle,
     LuCircleCheck,
@@ -250,6 +252,7 @@ export const AppIcon = {
     check: LuCheck,
     circle: LuCircle,
     clipboardCopy: LuClipboardCopy,
+    collapseAll: LuChevronsDownUp,
     collection: LuPackage2,
     delete: LuTrash,
     disc: LuDisc,
@@ -269,6 +272,7 @@ export const AppIcon = {
     emptySongImage: LuMusic,
     error: LuShieldAlert,
     expand: LuExpand,
+    expandAll: LuChevronsUpDown,
     externalLink: LuExternalLink,
     favorite: LuHeart,
     fileJson: LuFileJson,