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/store/offlineJobStore.ts

@@ -0,0 +1,46 @@
+import { create } from 'zustand';
+
+export interface DownloadJob {
+  trackId: string;
+  albumId: string;
+  albumName: string;
+  trackTitle: string;
+  trackIndex: number;
+  totalTracks: number;
+  status: 'queued' | 'downloading' | 'done' | 'error';
+}
+
+interface OfflineJobState {
+  jobs: DownloadJob[];
+  bulkProgress: Record<string, { done: number; total: number }>;
+  cancelDownload: (albumId: string) => void;
+  cancelAllDownloads: () => void;
+}
+
+// Module-level cancellation set — checked by downloadAlbum before each batch.
+export const cancelledDownloads = new Set<string>();
+
+export const useOfflineJobStore = create<OfflineJobState>()((set, get) => ({
+  jobs: [],
+  bulkProgress: {},
+
+  cancelDownload: (albumId) => {
+    cancelledDownloads.add(albumId);
+    // Remove queued (not yet started) jobs immediately so the counter drops.
+    set(state => ({
+      jobs: state.jobs.filter(j => !(j.albumId === albumId && j.status === 'queued')),
+    }));
+  },
+
+  cancelAllDownloads: () => {
+    const unique = [...new Set(
+      get().jobs
+        .filter(j => j.status === 'queued' || j.status === 'downloading')
+        .map(j => j.albumId),
+    )];
+    unique.forEach(id => cancelledDownloads.add(id));
+    set(state => ({
+      jobs: state.jobs.filter(j => j.status !== 'queued'),
+    }));
+  },
+}));