feat(desktop): add offline cache and verified app updates

Clean release notes in Android and desktop update dialogs. Persist downloaded audio and offline playback events per server and user; add desktop update checks, checksum verification and installer launch.

apps/web/src/desktop.test.ts

@@ -1,12 +1,29 @@
 import { afterEach, describe, expect, it, vi } from 'vitest';
+import { AuthSession, type AuthUser } from '@pipu/core';
 
-import { createDesktop, isDesktop } from './desktop';
+import { createDesktop, desktopFetch, isDesktop } from './desktop';
 
 afterEach(() => {
   vi.unstubAllGlobals();
 });
 
 describe('desktop bridge', () => {
+  it('restores the last desktop user when the proxy cannot reach the server', async () => {
+    vi.stubGlobal('fetch', vi.fn().mockResolvedValue(new Response('offline', { status: 502, headers: { 'x-pipu-transport-error': '1' } })));
+    const user = { id: 'alice', username: 'alice' } as AuthUser;
+    const clear = vi.fn();
+    const session = new AuthSession({ clientType: 'desktop', fetch: desktopFetch, baseUrl: 'http://local', refreshStore: {
+      get: async () => 'refresh', set: vi.fn(), clear, getUser: async () => user,
+    } });
+    expect(await session.restore()).toEqual({ status: 'signed_in', offline: true, user });
+    expect(clear).not.toHaveBeenCalled();
+  });
+  it('reports a failed proxy hop as a network failure, preserving real HTTP errors', async () => {
+    vi.stubGlobal('fetch', vi.fn().mockResolvedValue(new Response('offline', { status: 502, headers: { 'x-pipu-transport-error': '1' } })));
+    await expect(desktopFetch('http://local/api')).rejects.toThrow('Server unavailable');
+    vi.stubGlobal('fetch', vi.fn().mockResolvedValue(new Response('denied', { status: 401 })));
+    expect((await desktopFetch('http://local/api')).status).toBe(401);
+  });
   it('is inactive in the browser', () => {
     expect(isDesktop()).toBe(false);
   });