When Zed was downloading an update and the machine went to sleep, the
download would hang indefinitely on wake because the in-flight TCP
connection had silently died and nothing ever timed it out or retried.
This adds an inactivity `read_timeout` to the HTTP client so a stalled
response body errors out instead of hanging forever
(slow-but-progressing downloads are unaffected, since the timeout resets
on each chunk). It also promotes `App::on_system_wake` to a
multi-subscriber `Subscription` API and uses it in the auto-updater to
cancel an interrupted check/download on wake and start a fresh attempt.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fix hanging Zed update downloads after system sleep