Orbit desktop app: no in-app update affordance (check-for-updates / auto-update)

**Type:** enhancement
**Reported via:** beta feedback, Orbit desktop app v0.4.1 (Electron 41.7.1, macOS arm64)

A beta tester noted there's no way to update Orbit from inside the desktop app -- no "update" / "check for updates" button and no auto-update notification anywhere in the UI. To get a newer build they have to re-download and reinstall manually.

> "update button inside orbit environment is not there, would be nice to have it within orbit desktop app."

Orbit ships as installable packages (#43) but the running app has no self-update path: it doesn't tell the user when a newer release exists, and there's no in-app action to fetch and apply it. For a fast-moving beta where fixes land frequently, testers can silently sit on stale builds (we've repeatedly seen already-fixed bugs re-reported from older versions).

### Ask
Add an in-app update affordance:
- surface when a newer release is available (against the same GitHub release feed `release.yml` publishes), and
- a one-click "update / restart to update" action, or at minimum a "check for updates" menu item pointing the user at the new build.

Electron's `autoUpdater` / `update.electronjs.org` (or electron-updater) plus the existing Squirrel/zip makers are the usual mechanisms.

### Related
- #178 (`/orbit` v2 install-when-missing -- same release feed, but first-install from the CLI rather than in-app self-update)
- #43 (closed -- ship installable packages for macOS/Linux/Windows)
- #343 (Windows Squirrel packaging -- relevant to the Windows update channel)

- #348 (HID-resolution ergonomics) -- surfaced from the same beta feedback row

Start by reading the existing release.yml feed and the packaging work from #43 and #343, then review Electron's autoUpdater or update.electronjs.org options against the existing Squirrel/zip makers. Done means the app detects a newer release and provides an in-app update or check-for-updates path, with behavior suitable for the supported platforms.

Written by the indexing model from the issue text.