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/fr.ts

@@ -1267,6 +1267,7 @@ export const frTranslation = {
     deleteSuccess: '{{count}} playlists supprimées',
     deleteFailed: 'Erreur lors de la suppression de {{name}}',
     deleteSelected: 'Supprimer la sélection',
+    deleteSelectedPartial: 'Seulement {{n}} des {{total}} playlists sélectionnées peuvent être supprimées (les autres appartiennent à quelqu\'un d\'autre).',
     mergeSuccess: '{{count}} morceaux fusionnés dans {{playlist}}',
     mergeNoNewSongs: 'Aucun nouveau morceau à ajouter',
     mergeError: 'Erreur lors de la fusion des playlists',