fix(electron): preserve changelog on download + reliable restart-to-update

- useUpdateStore: keep sidecar-sourced body when merging fresh desktopInfo
  (electron-updater returns 'See release notes at ...' which clobbered it)
- main.mjs: defer quitAndInstall/relaunch via setImmediate so IPC reply
  flushes first; wire update-downloaded and error events; log restart path

packages/ui/src/stores/useUpdateStore.ts

@@ -244,6 +244,10 @@ export const useUpdateStore = create<UpdateStore>()((set, get) => ({
           ? {
             ...state.info,
             ...desktopInfo,
+            // Keep the richer sidecar-sourced changelog; desktopInfo.body is
+            // often the bare "See release notes at..." fallback from the
+            // updater and would otherwise clobber the nice changelog.
+            body: state.info.body || desktopInfo.body,
             available: state.info.available,
           }
           : desktopInfo,