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/README

@@ -90,6 +90,38 @@ Without these the jobs still build but produce unsigned/untrusted output. Becaus
 a public-history fork, add environment/branch protection so only release/healthchat + healthchat-v*
 runs can reach the signing credentials.
 
+Auto-update + public installer hosting
+--------------------------------------
+The apps self-update over a feed hosted on healthchat.ch (the GitHub Release is private to this
+internal fork, so it can serve neither end users nor the Squirrel auto-updater). Both the update
+feed and the public installers live under https://healthchat.ch/packages/desktop/ (FTP web root
+/public_html/healthchat.ch maps to https://healthchat.ch/; no packages. subdomain, so it's a path):
+
+  update/   the Squirrel feed the installed apps poll hourly. config.json sets
+              "update_base_url": "https://healthchat.ch/packages/desktop/update/"
+            and the macos job passes base-url: https://healthchat.ch/packages/desktop so the
+            generated releases.json points the *-mac.zip at this origin.
+              update/macos/releases.json + HealthChat-<ver>-mac.zip   (Squirrel.Mac)
+              update/win32/<arch>/RELEASES + HealthChat-<ver>-full.nupkg   (Squirrel.Windows; x64 + arm64)
+  install/  public downloads for the FIRST install (users get these URLs, not the GitHub Release):
+              install/macos/HealthChat-<ver>.dmg + HealthChat.dmg (stable "latest")
+              install/win32/<arch>/HealthChat-Setup-<ver>.exe + HealthChat-Setup.exe (stable) + *.msi
+
+The `release` job in build-healthchat-desktop.yaml stages these trees and uploads them via lftp/FTPS
+(mirror -R, no-delete so prior nupkgs survive for delta history). Add these repo secrets:
+  FTP_SERVER     hostname only (explicit FTPS / AUTH TLS on port 21; if the host cert is untrusted,
+                 flip ssl:verify-certificate to no in the workflow)
+  FTP_USERNAME
+  FTP_PASSWORD
+
+Caveats:
+  - First install is manual. Apps already installed without update_base_url (<= 1.1.2) cannot
+    self-update to the first build that carries it; users must download once from the install/ URL.
+    Every release after that updates automatically.
+  - Windows auto-update only works for the Squirrel Setup.exe install path, NOT the per-machine
+    .msi (the updater needs Squirrel's Update.exe, which the MSI doesn't install). Hand users the
+    Setup.exe as the primary Windows installer. macOS: install the .dmg into /Applications.
+
 Open items
 ----------
   - api_base_url: confirm the production API origin in config.json (currently assumed
@@ -99,5 +131,3 @@ Open items
     rewrites any `version` override into a nightly string), NOT the healthchat-v tag. Bump
     apps/desktop/package.json in the release commit if you want the About dialog / installer
     filenames to match the tag.
-  - Auto-update: not wired up. The apps install from the GitHub Release but do not self-update.
-    Adding it needs a hosted update feed (releases.json / RELEASES) on a stable URL.

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",