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/zipDownloadStore.ts

@@ -0,0 +1,46 @@
+import { create } from 'zustand';
+
+export interface ZipDownload {
+  id: string;
+  filename: string;
+  bytes: number;
+  /** null = Content-Length unbekannt (Navidrome on-the-fly ZIP) */
+  total: number | null;
+  done: boolean;
+  error: boolean;
+}
+
+interface ZipDownloadState {
+  downloads: ZipDownload[];
+  start: (id: string, filename: string) => void;
+  updateProgress: (id: string, bytes: number, total: number | null) => void;
+  complete: (id: string) => void;
+  fail: (id: string) => void;
+  dismiss: (id: string) => void;
+}
+
+export const useZipDownloadStore = create<ZipDownloadState>((set) => ({
+  downloads: [],
+
+  start: (id, filename) => set(state => ({
+    downloads: [...state.downloads, { id, filename, bytes: 0, total: null, done: false, error: false }],
+  })),
+
+  updateProgress: (id, bytes, total) => set(state => ({
+    downloads: state.downloads.map(d =>
+      d.id === id ? { ...d, bytes, total: total ?? d.total } : d
+    ),
+  })),
+
+  complete: (id) => set(state => ({
+    downloads: state.downloads.map(d => d.id === id ? { ...d, done: true } : d),
+  })),
+
+  fail: (id) => set(state => ({
+    downloads: state.downloads.map(d => d.id === id ? { ...d, error: true } : d),
+  })),
+
+  dismiss: (id) => set(state => ({
+    downloads: state.downloads.filter(d => d.id !== id),
+  })),
+}));