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.ts

@@ -14,11 +14,19 @@ export function isDesktop(): boolean {
   return api() !== undefined;
 }
 
-function invoke<T>(cmd: string, args?: Record<string, unknown>): Promise<T> {
+/** A failed loopback hop must behave like a network failure to offline auth restore. */
+export const desktopFetch: typeof fetch = async (input, init) => {
+  const response = await fetch(input, init);
+  if (response.headers.get('x-pipu-transport-error') === '1') throw new TypeError('Server unavailable');
+  return response;
+};
+
+export function desktopInvoke<T>(cmd: string, args?: Record<string, unknown>): Promise<T> {
   const tauri = api();
   if (!tauri) throw new Error('Tauri API unavailable');
   return tauri.core.invoke(cmd, args) as Promise<T>;
 }
+const invoke = desktopInvoke;
 
 export type DesktopHandle = {
   proxyBase: string;