feat(desktop): wire up HealthChat auto-update + public installer hosting

The Electron auto-updater (Squirrel.Mac / Squirrel.Windows) was already
implemented but switched off for HealthChat because the variant config
omitted update_base_url, and CI published only installers (no update feed),
to a private GitHub Release that can't serve end users or the updater.

- config.json: set update_base_url to https://healthchat.ch/packages/desktop/update/
  so electron-main starts the updater.
- build-healthchat-desktop.yaml: pass base-url to the macOS job so releases.json
  is generated; extend the release job to stage the update feed + public
  installers and mirror them to healthchat.ch over FTPS (lftp, no-delete to
  preserve prior nupkgs; password via LFTP_PASSWORD).
- Document the feed/install layout, FTP secrets, and the two caveats (first
  install is manual; Windows auto-update needs the Setup.exe path, not .msi)
  in the healthchat README and the releasing-healthchat skill.

Requires repo secrets FTP_SERVER / FTP_USERNAME / FTP_PASSWORD.

apps/desktop/element.io/healthchat/config.json

@@ -96,6 +96,7 @@
         ]
     },
     "show_labs_settings": false,
+    "update_base_url": "https://healthchat.ch/packages/desktop/update/",
     "modules": ["/modules/healthchat-module.js"],
     "ch.healthchat": {
         "api_base_url": "https://healthchat.ch",