Desktop: Add desktop auto-updates

[priethor]

What

Adds in-place updates for the signed macOS desktop app. Cortext now checks published GitHub Releases, downloads a newer build when one is available, and asks the user to restart once it is ready. The app menu gets update controls too. After an app update, Cortext refreshes the bundled WordPress/Cortext files in the extracted site without touching the user's database or uploads.

Why

Desktop releases should not require a fresh DMG install every time. We also need the extracted WordPress site to pick up the app's bundled code without wiping local data.

How

• Publishing the mac update feed next to the DMG on GitHub Releases.
• Tying download/install behavior to one app-menu setting.
• Refreshing bundled site files after an app update while carrying user data forward.

Testing Instructions

1. Build a signed desktop release and confirm the DMG, mac zip, zip blockmap, and latest-mac.yml are present.
2. Install Cortext in Applications.
3. Publish a newer test Release and choose Cortext > Check for Updates.
4. Confirm Cortext downloads the update and asks to restart.
5. Restart into the new version and confirm existing local content and uploads are still there.