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/en.ts

@@ -17,6 +17,7 @@ export const enTranslation = {
     collapse: 'Collapse Sidebar',
 
     downloadingTracks: 'Caching {{n}} tracks…',
+    cancelDownload: 'Cancel download',
     offlineLibrary: 'Offline Library',
     genres: 'Genres',
     playlists: 'Playlists',
@@ -824,6 +825,7 @@ export const enTranslation = {
     addSong: 'Add to playlist',
     cacheOffline: 'Cache playlist offline',
     offlineCached: 'Playlist cached',
+    removeOffline: 'Remove from offline cache',
     offlineDownloading: 'Caching… ({{done}}/{{total}} albums)',
     publicLabel: 'Public',
     privateLabel: 'Private',