feat(playlists): local playlist folders (sidebar + page, DnD, view toggle) (#1119)

* feat(playlists): playlist folder model — store + pure grouping core

Local, per-server folder layer over the server's flat playlist list (the
Subsonic API has no folder concept). Adds:
- playlistFolders.ts: shared types + pure groupPlaylistsByFolder (used by
  every surface), with full unit coverage.
- playlistFolderStore.ts: persisted Zustand store (create/rename/delete/
  assign/collapse), per-server scoped; deleting a folder drops its
  assignments so playlists fall back to ungrouped.

UI surfaces (sidebar + Playlists page) build on this in following commits.

* i18n(playlists): folder strings across all 9 locales

Adds the playlists.folders.* namespace (folder names, move/remove, expand/
collapse, group-by-folders toggle, count plurals, and the local-only notice
explaining that Navidrome and the Subsonic API have no native folder support).

* feat(playlists): folder views, drag-to-folder, move-to-folder menu, view toggle

Surfaces the local folder layer on both the Playlists page and the sidebar:
- Page: collapsible folder sections + ungrouped remainder, each reusing
  VirtualCardGrid; flat grid is kept verbatim when no folders exist or the
  group view is toggled off.
- Drag-to-folder via the shared mouse-based psy-drop system (HTML5 DnD is
  unusable in WebKitGTK); the whole section is the drop zone, and the
  ungrouped zone appears during a drag so a playlist can always return to root.
- "Move to folder" submenu in the playlist context menu (keyboard-accessible
  path; also creates folders on the fly) — stays available offline.
- Header gets a "New folder" action and a "Group by folders" view toggle
  (both context-aware); a notice surfaces the local-only caveat.
- Sidebar renders the same collapsible folder groups.
- groupView preference added to the folder store.

* docs(changelog): note playlist folders (#1119)

src/locales/nl/playlists.ts

@@ -101,4 +101,22 @@ export const playlists = {
   csvImportToast: '{{added}} toegevoegd, {{notFound}} niet gevonden, {{duplicates}} duplicaten',
   csvImportDownloadSuccess: 'Rapport succesvol gedownload',
   csvImportDownloadError: 'Rapport downloaden mislukt',
+  // Playlist folders (local, per-server organisation layer)
+  folders: {
+    newFolder: 'Nieuwe map',
+    namePlaceholder: 'Mapnaam…',
+    create: 'Aanmaken',
+    rename: 'Hernoemen',
+    delete: 'Map verwijderen',
+    ungrouped: 'Zonder map',
+    moveToFolder: 'Naar map verplaatsen',
+    removeFromFolder: 'Uit map verwijderen',
+    expandFolder: 'Map uitvouwen',
+    collapseFolder: 'Map invouwen',
+    groupByFolders: 'Groeperen op mappen',
+    count_one: '{{count}} playlist',
+    count_other: '{{count}} playlists',
+    localOnlyNotice:
+      'Mappen worden alleen in Psysonic op dit apparaat opgeslagen. Navidrome en de Subsonic-API ondersteunen geen playlistmappen, dus de structuur wordt niet op je server opgeslagen of met je andere apparaten en apps gesynchroniseerd.',
+  },
 };