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/ru.ts

@@ -1346,6 +1346,7 @@ export const ruTranslation = {
     deleteSuccess: '{{count}} плейлистов удалено',
     deleteFailed: 'Ошибка при удалении {{name}}',
     deleteSelected: 'Удалить выбранные',
+    deleteSelectedPartial: 'Можно удалить только {{n}} из {{total}} выбранных плейлистов (остальные принадлежат другим пользователям).',
     mergeSuccess: '{{count}} треков объединено в {{playlist}}',
     mergeNoNewSongs: 'Нет новых треков для добавления',
     mergeError: 'Ошибка при объединении плейлистов',