feat(playlists): add ZIP download button to playlist detail page

Mirrors the album download UX: Download (ZIP) button in the playlist
hero header, progress bar while downloading, folder picker via
downloadModalStore (remembers last used folder). Uses the same
Subsonic /rest/download.view endpoint as album downloads — it accepts
playlist IDs just as well as album IDs.

Closes #127

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/locales/zh.ts

@@ -771,6 +771,7 @@ export const zhTranslation = {
     removeCover: '删除照片',
     coverUpdated: '封面已更新',
     metaSaved: '播放列表已更新',
+    downloadZip: '下载 (ZIP)',
   },
   radio: {
     title: '网络电台',