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/i18n/locales/en.json

@@ -6,6 +6,8 @@
         "selectRangeOfItems": "Select a range of items",
         "clearQueue": "Clear queue",
         "goToCurrent": "Go to current item",
+        "collapseAllFolders": "Collapse all folders",
+        "expandAllFolders": "Expand all folders",
         "createPlaylist": "Create $t(entity.playlist, {\"count\": 1})",
         "createRadioStation": "Create $t(entity.radioStation, {\"count\": 1})",
         "deletePlaylist": "Delete $t(entity.playlist, {\"count\": 1})",
@@ -56,6 +58,7 @@
         "albumPeak": "Album peak",
         "areYouSure": "Are you sure?",
         "ascending": "Ascending",
+        "back": "Back",
         "backward": "Backward",
         "biography": "Biography",
         "bitDepth": "Bit depth",
@@ -1044,6 +1047,19 @@
         "sidebarConfiguration": "Sidebar configuration",
         "playerItemConfiguration_description": "Configure what items are shown, and in what order, on the fullscreen player",
         "playerItemConfiguration": "Player item configuration",
+        "sidebarPlaylistFolders_description": "Create a folder view for playlists that include the configured separator in the name",
+        "sidebarPlaylistFolders": "Enable folders",
+        "sidebarPlaylistFolderSeparator_description": "Character (or string) that separates folder levels in a playlist name",
+        "sidebarPlaylistFolderSeparator": "Folder separator",
+        "sidebarPlaylistFolderView_description": "How folders are displayed in the sidebar",
+        "sidebarPlaylistFolderView": "Folder view",
+        "sidebarPlaylistFolderView_optionSingle": "Single folder",
+        "sidebarPlaylistFolderView_optionTree": "Tree view",
+        "sidebarPlaylistFolderView_optionNavigation": "Navigation view",
+        "sidebarPlaylistFolderTreeIndent_description": "Pixels each tree level is indented",
+        "sidebarPlaylistFolderTreeIndent": "Tree indent",
+        "sidebarPlaylistFolderTreeLineColor_description": "Color of the connecting tree lines (leave empty for theme default)",
+        "sidebarPlaylistFolderTreeLineColor": "Tree line color",
         "sidebarPlaylistList_description": "Show or hide the playlist list in the sidebar",
         "sidebarPlaylistList": "Sidebar playlist list",
         "sidebarPlaylistSorting_description": "Allows manual playlist sorting in the sidebar using drag and drop instead of the default server order",