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/nb.ts

@@ -16,6 +16,7 @@ export const nbTranslation = {
     expand: 'Utvid sidefelt',
     collapse: 'Skjul sidefelt',
     downloadingTracks: 'Bufre {{n}} spor…',
+    cancelDownload: 'Avbryt nedlasting',
     offlineLibrary: 'Frakoblet bibliotek',
     genres: 'Sjangere',
     playlists: 'Spillelister',
@@ -820,6 +821,7 @@ export const nbTranslation = {
     addSong: 'Legg til i spilleliste',
     cacheOffline: 'Bufre spilleliste offline',
     offlineCached: 'Spilleliste bufret',
+    removeOffline: 'Fjern fra offline-buffer',
     offlineDownloading: 'Bufre… ({{done}}/{{total}} album)',
     publicLabel: 'Offentlig',
     privateLabel: 'Privat',