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/zh.ts

@@ -1261,6 +1261,7 @@ export const zhTranslation = {
     deleteSuccess: '{{count}} 个播放列表已删除',
     deleteFailed: '删除 {{name}} 时出错',
     deleteSelected: '删除所选',
+    deleteSelectedPartial: '所选 {{total}} 个播放列表中只有 {{n}} 个可删除（其余属于他人）。',
     mergeSuccess: '{{count}} 首歌曲已合并到 {{playlist}}',
     mergeNoNewSongs: '没有新歌曲可添加',
     mergeError: '合并播放列表时出错',