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/de.ts

@@ -16,6 +16,7 @@ export const deTranslation = {
     expand: 'Sidebar einblenden',
     collapse: 'Sidebar ausblenden',
     downloadingTracks: '{{n}} Tracks werden gecacht…',
+    cancelDownload: 'Download abbrechen',
     offlineLibrary: 'Offline-Bibliothek',
     genres: 'Genres',
     playlists: 'Playlists',
@@ -823,6 +824,7 @@ export const deTranslation = {
     addSong: 'Zur Playlist hinzufügen',
     cacheOffline: 'Playlist offline speichern',
     offlineCached: 'Playlist gecacht',
+    removeOffline: 'Aus Offline-Cache entfernen',
     offlineDownloading: 'Wird gecacht… ({{done}}/{{total}} Alben)',
     publicLabel: 'Öffentlich',
     privateLabel: 'Privat',