fix(playlists): surface bulk-delete button in header while multi-select is active (#290)

Selecting playlists via the header toggle only exposed the delete action
through the right-click context menu. Add a visible Delete-selected button
next to Cancel that kicks off the same handleDeleteSelected flow, with a
tooltip hint when some of the selected playlists aren't deletable (foreign
owner). Button is only rendered when selection mode is on and at least one
playlist is selected.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/locales/es.ts

@@ -1272,6 +1272,7 @@ export const esTranslation = {
     deleteSuccess: '{{count}} listas eliminadas',
     deleteFailed: 'Error al eliminar {{name}}',
     deleteSelected: 'Eliminar seleccionadas',
+    deleteSelectedPartial: 'Solo {{n}} de {{total}} listas seleccionadas pueden eliminarse (las demás pertenecen a otro usuario).',
     mergeSuccess: '{{count}} canciones unificadas en {{playlist}}',
     mergeNoNewSongs: 'No hay canciones nuevas para agregar',
     mergeError: 'Error al unificar listas',