feat: fix UI freezes in ZIP and offline cache downloads

ZIP downloads (PlaylistDetail, Albums, NewReleases, RandomAlbums) now use
invoke('download_zip') via Rust streaming instead of fetch+blob+arrayBuffer,
eliminating JS heap saturation on large files. Progress shown in ZipDownloadOverlay.

Offline cache downloads (600+ songs) no longer freeze the UI: transient job
state moved to a separate non-persisted offlineJobStore, reducing localStorage
writes from ~1200 down to 2 for an entire download. Also adds playlist offline
toggle — clicking the cache button when already cached removes it from cache.

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

src/locales/zh.ts

@@ -16,6 +16,7 @@ export const zhTranslation = {
     expand: '展开侧边栏',
     collapse: '收起侧边栏',
     downloadingTracks: '正在缓存 {{n}} 首歌曲…',
+    cancelDownload: '取消下载',
     offlineLibrary: '离线音乐库',
     genres: '流派',
     playlists: '播放列表',
@@ -817,6 +818,7 @@ export const zhTranslation = {
     addSong: '添加到播放列表',
     cacheOffline: '离线缓存播放列表',
     offlineCached: '播放列表已缓存',
+    removeOffline: '从离线缓存中移除',
     offlineDownloading: '正在缓存… ({{done}}/{{total}} 张专辑)',
     publicLabel: '公开',
     privateLabel: '私有',