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/nl.ts

@@ -16,6 +16,7 @@ export const nlTranslation = {
     expand: 'Zijbalk uitklappen',
     collapse: 'Zijbalk inklappen',
     downloadingTracks: '{{n}} nummers worden gecached…',
+    cancelDownload: 'Download annuleren',
     offlineLibrary: 'Offline bibliotheek',
     genres: 'Genres',
     playlists: 'Playlists',
@@ -821,6 +822,7 @@ export const nlTranslation = {
     addSong: 'Toevoegen aan playlist',
     cacheOffline: 'Playlist offline opslaan',
     offlineCached: 'Playlist gecached',
+    removeOffline: 'Verwijder uit offline cache',
     offlineDownloading: 'Cachen… ({{done}}/{{total}} albums)',
     publicLabel: 'Openbaar',
     privateLabel: 'Privé',