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-tauri/Cargo.toml

@@ -35,7 +35,7 @@ symphonia = { version = "0.5", default-features = false, features = ["flac", "mp
 reqwest = { version = "0.12", default-features = false, features = ["stream", "json", "multipart", "rustls-tls", "blocking"] }
 futures-util = "0.3"
 md5 = "0.7"
-tokio = { version = "1", features = ["rt", "time"] }
+tokio = { version = "1", features = ["rt", "time", "sync"] }
 biquad = "0.4"
 ringbuf = "0.3"
 tauri-plugin-window-state = "2.4.1"